The General Purpose Cartridge Revisited

I've come under some fire on Internet forums for "misrepresenting" the GPC concept as being heavier than it would in actuality. While I feel that the concept in my The Case Against article - though constrained by several requirements set forth by some members on Mr. Williams' forum - is a perfectly valid approximation, today I hope to address this complaint by creating the absolute lightest GPC estimate I possibly can while still sticking to conventional ammunition design and fulfilling the basic requirement that it not have any less residual energy at 1,000 m than M80 Ball.

First, I need to recall that M80 produces just over 410 J at that range (.195 G7 BC, 2,750 ft/s muzzle velocity, Army Metro atmospheric conditions). You can see this for yourself if you head over to JBM's ballistic calculator and plug in the appropriate values. My GPC must meet or exceed this value. To really get the lightest weight possible, I'm going to need a very good form factor for my bullet- since this is all about retained energy at range. The best form factor I know of for a service projectile is .889 i7, and belongs (paradoxically) to the first spitzer rifle bullet to see service, the French Balle D, an all-brass 12.8 gram boattail projectile with a fairly unique design. If I scale that bullet down to .264" caliber, it weighs just under 7.2 grams.

Since the Army has spent a bunch of time, money, and effort refining their M855A1 EPR design, it seems unlikely that any new caliber would have a substantially different projectile design. Fortunately, I only need to make moderate changes to the Balle D to accommodate this. The scaled-down projectile will be slightly longer - proportionally - than the original at 33.5mm in length, as well as slightly lighter at 7.0 grams. Now, I can find my ballistic coefficient, which works out to a very impressive .249 G7. Running that through JBM a few times tells me I need a muzzle velocity of 2,800 ft/s to meet M80's energy  at a kilometer:

These are some very impressive ballistics! Now, let's see how much this performer would weigh. To shave as much weight as possible from the design, I am going to use M855A1's peak pressure of 62,000 PSI, as well as the same case taper and shoulder angle of 5.56. Many have argued that 62,000 PSI is too much pressure for a rifle cartridge, and that 5.56's case taper and shoulder angle are a detriment to its reliability in automatic weapons, which is part of the reason why my last estimate was so heavy. This time around, I'm pulling out all the stops; no expense will be spared making this GPC as light as I can. In my opinion, the .5 degrees case taper and 23 degree shoulder of the 5.56mm are adequate, and the 62,000 PSI peak pressure is made safe by more thermally stable propellants. I will move on.

After doing several run throughs with the Powley computer, I found that about 41 grains case capacity was needed to generate the sort of performance I was looking for from 20" barrels (this, as far as I know, being the length of barrel chosen by Mr. Williams as optimum for his GPC). Using Solidworks, I created a fairly svelte case: based on the .30 Remington case head and only 46.4mm long, it has almost exactly 41 grains case capacity:

Here are the Powley results for my completed cartridge:

Judging by the propellant weights of both the 6.8 SPC and the 6.5 Grendel, though, that 29.1 grain charge weight seems a little light to me. A spit-in-the-wind estimate puts me at 34.7 grains charge, which sounds more reasonable. Now, we can figure out how much the cartridge weighs.

The bullet, of course, will be 7.00 grams. Large rifle primers weigh about .35 grams, as I mentioned in The Case Against. The charge, in metric, works out to 2.25 grams, and the case itself weighs 7.39 grams if made of brass, or 6.91 grams if made of steel. I will assume steel-cased ammunition, which gives me a total weight of 16.51 grams per cartridge (16.99 grams for brass). 16.5 grams is absolutely fantastic, and lighter even than Mr. Williams' target weight for a GPC with a 7 gram bullet. Let's look at what that means for the infantry platoon:

Weight of ammunition (only, not including magazines or links) of infantry squad breakdown by squad role: 

Squad Leader: 210 rounds 5.56mm, totaling 2.54 kg
Team Leader: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totalling 4.96 kg (x2)
Automatic Rifleman: 800 rounds linked 5.56mm, totaling 9.68 kg (x2)
Grenadier: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totaling 4.96 kg (x2)
Rifleman: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totaling 4.96 kg (x2) 

Weight of ammunition of infantry squad: 51.7 kg (x3) 

Weight of ammunition of infantry squad with 16.5 gram GPC: 70.5 (x3) 

Weight of ammunition of infantry squad with 20 gram GPC: 85.4 kg (x3) 

Weight of ammunition of infantry squad with M240s in place of M249s: 85.5 kg (x3)

Weight of ammunition (only, not including magazines or links) of weapons squad breakdown by squad role: 

Squad Leader: 210 rounds 5.56mm, totaling 2.54kg
Machine Gunner: 300 rounds linked 7.62mm, totaling 7.26 kg (x2)
Assistant Gunners: 210 rounds 5.56mm in magazines, 400 rounds linked 7.62mm for M240 totaling 12.22 kg (x2)
Ammunition Bearer: 210 rounds 5.56mm in magazines, 300 rounds linked 7.62mm for M240, 140 rounds 7.62mm in magazines totaling 13.2 kg (x2) 

Weight of ammunition of weapons squad: 64.5 kg  

Weight of ammunition of weapons squad with 16.5 gram GPC: 53.0 kg

Weight of ammunition of weapons squad with 20 gram GPC: 63.8 kg

Weight of ammunition (only, not including magazines or links) of platoon headquarters breakdown by squad role 

Platoon Leader: 210 rounds 5.56mm, totaling 2.54 kg
Platoon Sergeant: 210 rounds 5.56mm, totaling 2.54 kg
Radio Operator: 210 rounds 5.56mm, totaling 2.54 kg
Combat Medic: 210 rounds 5.56mm, totaling 2.54 kg 

Weight of ammunition of platoon headquarters: 10.2 kg  

Weight of ammunition of platoon headquarters with 16.5 gram GPC: 13.4 kg

Weight of ammunition of platoon headquarters with 20 gram GPC: 16.8 kg 

  

Total weight of ammunition in the platoon: 229.3 kg    

Total weight of ammunition in the platoon with 16.5 gram GPC: 277.9 kg

Total weight of ammunition in the platoon with 20 gram GPC: 336.8 kg 

Total weight of ammunition in the platoon with M240s in place of M249s: 330.9 kg 

Weight increase in the platoon if Carl Gustafs are issued in place of 60mm mortars: 47.5 kg

As my readers can see, even this extremely optimistic estimate still adds almost fifty kilograms of ammunition to the infantry platoon (to say nothing of the increase in weight from the heavier belt links, magazines, and weapons that would be needed to chamber the cartridge). I've heard some GPC proponents claim that the savings in weight of a GPC vs. 7.62mm would balance out its increased weight vs. 5.56mm weapons at the platoon level; this is clearly not true. The GPC - even the lightest possible GPC - represents a significant weight increase for the infantry rifle platoon (or a corresponding reduction in rounds carried). Further, even replacing the platoon's 60mm mortars with direct-fire recoilless rifles is lighter than switching whole hog to the GPC. The Army has already decided to introduce Carl Gustafs to the infantry, which in my mind neatly solves the issue of having to call in air or artillery support when under fire from long-range Taliban ambushes; this being the primary rationale for the GPC in the first place. If the mortars are left home, American soldiers will be able to project high explosive firepower at long ranges without the need for an expensive and time consuming ammunition change-over, and with no more additional burden at the platoon level than would result from fielding a GPC.

This cartridge I've designed is an impressive ballistic hot rod. It would make some sense for 7.62mm weapons to be replaced with weapons in a similar caliber, retaining the capability of that round while slashing the weight of ammunition by about 30%. I felt much the same in my initial article about my 20 gram GPC estimate. Anything that reduces the soldier's load without reducing capability or introducing problems is probably worth doing, if you can afford it. What I disagree with is the idea that a cartridge such as this should be a general purpose cartridge; that it should be issued to riflemen as well as designated marksmen, machine gunners, and as ammunition for static machine guns. The additional weight of ammunition and rifles, and increased recoil and size, sours the idea considerably in my mind vs. the two caliber system.

Called It!

I don't normally reblog without commentary, but it seems my earlier notion that the Carl Gustav could be issued in the general Army as a way to improve infantry effectiveness at long range has proven prescient.

.300 AAC Blackout: The End of 5.56?

This PDF from Advanced Armament Corp extolling the virtues of the .300 AAC Blackout vs. 5.56mm includes a curious section:

Boy those sure are some impressive numbers for an overgrown .30 Carbine! Do they reflect reality, though? Let's check JBM's ballistic calculator. First, we'll need to find a ballistic coefficient figure that reflects a 125gr .300 Blackout load. Here's a very impressive looking Sierra 125gr Matchking, but it's BC is in G1, not G7 like we want. Fortunately for us, JBM has a drag function converter. Using this, we get a G7 ballistic coefficient at 2,220 ft/s of .170. Now we can plug all this into JBM's trajectory calculator to see if AAC's claims are accurate. For this, we'll leave energy in ft-lbs, but change range to be in meters:

That's... Pretty much right on the money for AAC's claims. What about their numbers for 5.56mm?

Not even close, for trajectory.

Reevaluating this, if we take 5.56's performance at 500m as a baseline for "maximum effective range", then it out-ranges .300 AAC Blackout by about 110m. Further, it's worth noting that at 250m, 5.56 has less than a quarter of the drop, thanks to its laser-like trajectory, which enables the 25 yard combat zero. Worse, 5.56 actually generates slightly more energy per pound than the 125gr .300 AAC Blackout, with 23.8 ft-lbs/gram, vs. 22.7 ft-lbs/gram (how about those units? Working in firearms sure is fun!). It's also worth remembering that larger FMJs tumble less readily than smaller ones and will deposit less of their energy into a human target, something I can't seem to repeat often enough.

So does all this make the .300 AAC Blackout a bad cartridge? No, I don't think so. It seems like an interesting concept, being able to switch from a quiet subsonic cartridge to a harder-hitting and longer-ranged supersonic one without changing any hardware other than the magazine. Of course, the high speed low drag types in my audience will have to tell me whether that sort of concept is attractive to anyone other than readers of Tactical Life magazine, but it's certainly still worth looking at.

What this does remind us is to trust but verify any data you see in a manufacturer's pamphlet or public materials. In this case, it really does seem like AAC had no problem "massaging" the numbers to make their cartridge look better than it really is.

On Long Range Combat Shooting

Several times on Internet fora I have been challenged for a "discrepancy" in my position on small arms ballistics, that my claims of having shot man sized targets with 5.56mm weapons out to 900 yards didn't mesh with my assertion that infantrymen are going to have a very, very difficult time hitting anything beyond 500 meters. I've responded to this in various ways over the years (something I've found difficult, never having given military service myself), but here's Chris Hernandez on the matter (H/T, Weaponsman):

Second thing: For most modern combat, 300 meters is plenty far. I carried an M14EBR (Enhanced Battle Rifle) in Afghanistan, and I could consistently hit a torso-sized rock at 900 meters – at the range, with perfect weather conditions, a good firing position, on a stationary target at a known distance. In combat, with extreme heat or cold, unknown distances, hasty firing positions, adrenaline and moving targets, plus little annoyances like incoming fire, I would have been ecstatic to smoke a mofo at 200 meters.

Every now and again, the stars will align, and a (usually very good) shooter will get a chance to hit targets in combat beyond 500 meters, but this is very rare. Even so, specialized 5.56mm ammunition seems to perform decently at these ranges.

Go forth and read the whole article (which is about optics selection).

Accurate Barrel Life Calculator

Here's a barrel life calculator I picked up while in school. It estimates the useful barrel life of a cartridge, given the input parameters.

A few examples:

M855:

	Accurate Barrel Life
	Bullet dia [in]	0.223
	Loaded Powder [gr]	25.2
	Powder heat potential [KJ/Kg]	3880
	Pressure [Psi]	58000
	Moly Coating [Y/N]	n
	Total	3049

M80:

	Accurate Barrel Life
	Bullet dia [in]	0.308
	Loaded Powder [gr]	46
	Powder heat potential [KJ/Kg]	3990
	Pressure [Psi]	62000
	Moly Coating [Y/N]	n
	Total	2660

6.5/8/800:

	Accurate Barrel Life
	Bullet dia [in]	0.264
	Loaded Powder [gr]	36.3
	Powder heat potential [KJ/Kg]	3890
	Pressure [Psi]	55000
	Moly Coating [Y/N]	n
	Total	2951

It's important to note that this calculator is most useful for precision shooting, but I think in general the results can be multiplied by about 5 to gain an idea of the useful military life of a barrel. Of course, numerous factors influence how toasty a cartridge is to a barrel, including what the barrel's made of, how high the flame temperature of the powder is, and how the bullet is constructed, so this value should only be used to give the user an idea of how much their design will aggravate these factors.

Comparing 85gr TSX 6.8mm to 5.56

At the request of commenter Angus McThag, I will do a (short) comparison of the 6.8 SPC loaded with the 85gr Barnes TSX to simulated M855 and M193 loads. I already whipped up a simulated M855 load in my previous post about 5.56 replacements, and I will be referring to that regularly through this article, instead of duplicating the information here. In addition, I will not be using Solidworks models for this estimate, as a reasonable estimate can be attained without doing so.

In order to make the comparison as even as possible, I need to create a Powley computer performance estimate. While using the Powley computer may not necessarily give the most accurate numbers in terms of performance for a given cartridge, it does provide the most even basis I know of for comparison. In this case, our 85gr 6.8 SPC provides a somewhat sedate 2,770 ft/s with an 85gr bullet:

As you can see, the peak pressure in this example is only 55,000 PSI, vs. the 58,000 PSI used in my last comparison. It is lower because 55,000 PSI is the maximum average pressure of 6.8 SPC according to SAAMI. If 58,000 PSI is used, the cartridge gains about 30 ft/s. The value of 2,770 ft/s sounded a bit low to me, and I thought maybe the Powley computer was giving me an incorrect result. However, when I adjusted the barrel length to 24" and checked it against Hodgdon's reloading data, I found that it was only about 41 ft/s lower than their value. 41 ft/s is definitely enough to make a difference in the performance of a cartridge, but it doesn't imply some kind of error in the computer itself (individual shots can often vary by more than 40 ft/s). For the same of comparison, I will continue to use the 2,770 ft/s value, instead of adjusting it, while noting that real-world values may be slightly higher.

Plugging this into JBM Ballistics' calculator, we get this (G7 BC is from here):

Just over 300 J at 500m? Jeeze, that's not that great. As we found in my last post on the subject, M855 produces over 400 J at that same range. Well, OK, but which cartridge weighs more? Your first clue is the weight of their respective bullets. Having not rendered 6.8 SPC in Solidworks yet, I can't give you an exact figure, but using the weight from the 6.8 Tech Information page I linked earlier of 28 rounds of ammunition, I estimate that the 85gr 6.8 SPC loading would weigh over 15 grams. Given this, it compares pretty unfavorably to M855, producing only 20.5 J/g at 500m, compared to 35.5 for 5.56.

Why is this value so low? The 85gr TSX is a flat-based bullet, with a poor form factor. It's also pretty light for caliber, and has a fairly low sectional density (about the same as 55gr 5.56)). In light of that, maybe we should compare it to M193:

It only provides 4% more energy at 500m. That's... Rather disappointing. We know that M193 has a cartridge weight of 11.5g, giving it approximately 25.9 J/g at 500m, a full 26% better than the 85gr TSX 6.8. The TSX definitely has the edge in bullet design from a terminal effects standpoint, but those ballistic results simply aren't very impressive.

I don't think I'm very happy with this result. On the chance that the 2,770 ft/s figure is just too low, I tried to dig up the absolute best performance figures I could for the 85gr TSX, and see how they compared to my Powley estimate for M855. I didn't find much, but I did find David Fortier's article on LWRC's Six8 UICW and its special ATK-made 90gr Gold Dot ammunition. Close enough, I think. To get some sort of performance baseline, I did a little math and got an estimated value of 2,940 ft/s from a 14.5" barrel for the 85gr TSX (this is essentially estimating if you pulled the bullet on an ATK 90gr Gold Dot, and reloaded the case, powder and all, with an 85gr TSX, and then fired it from a 14.5" barrel. Of course, this assumes that the values in the article are correct, and I suspect they're a bit optimistic.). Now we're talking. At 500m, this is what it does:

It gains almost 60 J at range from the extra 170 ft/s muzzle velocity, but it still doesn't compare very well to either 5.56, producing only 24.3 J/g at 500m. That's not great performance for a "best case scenario" of the lightweight 6.8mms.

This article illustrates most clearly to me the value of high form factor, long ogive bullets. At the muzzle, the 6.8mm with lightweight bullets looks fairly impressive, but the poor form factors of those bullets really let it down after a half-kilometer. Even by 300m, the initial estimate for the TSX has fallen below M855 in terms of absolute energy, not to mention energy per pound. The 6.8mm cartridge leaves even less space than 5.56 for long ogive, slender bullets, and I think this is the key to understanding its fairly lackluster performance with low sectional density bullets.

How to remove and replace your Vz. 58's fire control group

This article was originally posted on another blog of mine in late 2010, but its content is more appropriate here, I think.

I'm going to deviate a little bit from the established nature of this blog, simply because I like to play Good Samaritan every so often and actually add useful content to the Internets.

You see, recently, I purchased a Vz. 58 steel trigger, to replace the plastic trigger my CZ-USA Vz. 58 came with. When I looked on the Internet for info on how to muck around inside the receiver, I found nothing, so I had to figure it out myself. Trying to remove the fire control group from this rifle without knowing how is really hard, and results in lots of unnecessary scrapes on your receiver, and a lot of fruitless banging and frustration. Removing the FCG after you know what to do, however, is really quite easy, if a bit tricky. I decided to write this to help people do it the easy way.

The parts can get a bit confusing. I use the same terminology Czechpoint does, and you can find a nearly complete parts list for the Vz. 58 at their website:
http://www.czechpoint-usa.com/products/spare-parts-and-accessories/vz-58-parts-and-accessories/
The title is not so much an accurate description of the content as it is all the search terms I used in trying to find info about how to perform surgery on my Vz.

Let's begin.

Firstly, and most importantly, you want to remove the magazine and clear the chamber.

Then, and this is why that last step was so important, you want to flick the safety off if it wasn't already, and pull the trigger.

If you followed step one, you should get a click. If you didn't, you should get temporary hearing loss and a hole in something.

Now, push out the receiver cover retaining pin. This pin likes to push out most of the way and leave about a fifth of itself still retaining the cover, so make sure it's all the way out. If you have difficulty removing the cover, this is almost certainly the problem.

Next, remove the receiver cover by pushing it forward from the rear, and lifting it up. It should come straight on out. Note the dual captive springs; it's important to line them up properly during reassembly.

Now, remove the bolt carrier assembly by retracting it all the way to the rear, and removing it from the top.

Finally, you can inspect the glory that is the Vz. 58 fire control group. When I first tried to remove the old trigger, I just banged on the trigger axis pin until I destroyed two pulled 7.62x54R bullets and gave up that approach. It turns out, the Czechs thought of their trigger axis and sear axis pins backing out and put in two little e-clips.

These e-clips hold the pins in despite the application of considerable hammering. These e-clips are simple to remove, but for God's sake do not lose them. Just take a screwdriver, or other fine implement, and flick them out. They come off readily.

Once both e-clips for the sear and trigger pins are off, tap the two pins out slightly from the left to the right. Tap the sear pin out first, then the trigger pin. Once they come out about a millimeter, there's enough space for you pull them out all the way with a screwdriver or your fingernails. They shouldn't fit tightly.

It'll take a little finagling, but it should be pretty easy to remove the sear.

Once you have, remove the trigger assembly. To do this, press the trigger up from the bottom, and compress the disconnector spring enough that the trigger and disconnector come free from the sear assembly.

Now you have the trigger assembly in your hands. To install your brand new trigger, simply tap out the disconnector pin holding the trigger and disconnector together, and remove the disconnector and disconnector spring.

Replace trigger, or work on the trigger, or make sweet love to it, or whatever you're going to do, and then prepare to go insane.

This next step is the only truly hard part in replacing the trigger on a Vz. 58. You must reassemble the trigger group, which means reinserting the disconnector spring, and then compressing the disconnector to just the right place that the holes line up, while holding the trigger assembly steady, and hammering the disconnector pin back into the assembly. You might be able to more easily do that with a pin vise, but I don't have one of those, so I enlisted my girlfriend to supply the requisite third hand, but found that the assembly is actually too small for three hands to fit on the blasted thing without me hammering our thumbs into pulp.

Instead, I found a pair of pliers, and that worked well enough. It's still a matter of trial and error, though, so be patient, and eventually the pin will work its way a little into the disconnector, and then it's just a simple task of tapping the little devil in gently.

Now that you have your trigger assembly back in one piece, you simply have to put it back in the way it came, and then pushing the pin back in. Do not put the e-clip back on the pin until you are 100% sure your trigger group works. I even went so far as to reassemble the entire rifle without the e-clips, just to make sure. Once the trigger is in there satisfactorily, start on the sear assembly.

It looks weird, but it's not difficult to do once you figure out what goes where. The sear assembly likes to come apart, especially that plastic thing, which I'm pretty sure is a replacement for an automatic mechanism, since it does basically nothing. Czechpoint doesn't sell this part, so if it breaks, you're SOL, I guess. Anyway, that do-nothing plastic thing fits with the large end toward the right, if your assembly came apart (mine did), and the sear fits with the long end basically sitting in the middle of the receiver, to hold the striker back before firing.

The sear group fits in so that the little tab jutting out from the bottom of the sear is pulled on by the disconnector. A little trial and error will make it fit properly. Interestingly, the sear and trigger pins are interchangeable, so don't worry that you got them mixed up.

Once your rifle's trigger works, simply slide the e-clips back onto the pins. This is actually easier than it sounds, if you use a screwdriver. They might not click, exactly, but if they look like they're on, they are.

Now you're ready to reassemble the rifle. Simply tilt the muzzle of the rifle downward, let the bolt flop forward in the carrier, and mate the carrier and receiver together at the cutouts in the rifle rails. Slide the bolt forward. Now if the sear engages the striker, that's a good sign. If it doesn't, you have more work to do. Remove the carrier and find out where you screwed up. If it holds the striker, pull the trigger, both to make sure the trigger group will release it properly and to prepare the partially assembled rifle to receive the cover.

Now take the bigger, top spring in the receiver cover, and align it with the topmost hole in the bolt carrier. slide it in, making sure the lower striker spring falls into place inside the striker (they call it a "linear hammer", since the actual firing pin is a separate part). Now slide the cover in most of the way.

Make sure the little tabs jutting down from the inside edge of the cover mate with the receiver, or it won't go on right. Once it's properly inserted, push in the retaining pin, and you're all set.