AR 15 Reloading – Do not let this happen to you.
Without the Throne
Article 3: Pressure Problems in AR 15, calibers other than 5.56
A short time ago I met a guy at the range that was shooting an AR 15 in 7.62×39. I was shooting an AR 15 in 264 LBC (aka 6.5 Grendel modified). After a while he had a stoppage that he could not clear. I had a look at the weapon, and discovered that one of the bolt lugs had sheared off and was stuck in the barrel extension. We then disassembled the upper from the lower. The guy was really pissed off at the manufacturer needless to say. I suggested that we first gather some information to use in his claim.
During our conversation I learned that he had only fired handloads he made (except for one box of S&B the dealer gave him) because he heard that steel cased ammo was junk, and would possibly break his weapon. Looking at the manufacturer’s literature showed that the warranty was void if handloads were used, so the fellow was out of luck with a claim.
This fellow indicated that he never loaded above any loadings that are listed in any of his handbooks, and he always worked up his loads in steps. His favorite load was WIN brass and 123 grain bullets over 29.5 grains of CFE BLK, which is the maximum listed by Hodgdon. He was very proud that he was getting 2520 fps out of his loads. This was in a 16” barrel. The factory cartridges he had tried only gave him 2320 fps, so this CFE BLK was really good stuff.
While we talked, he mentioned that when he was working up his load, he noticed that his primers (WIN WLR) were flattening a little at 28.6 grains, so he switched to CCI 250. He had heard they were thicker/harder so he thought he would try. Anyways this is a military caliber, and they use hard primers. Guess what, it worked, he could use the max charge and the primers were no flatter than the 28.6 grain one.
I suggested that his loads were well over pressure and if he was interested, I would explain why. That is what this article is about.
Why did this Happen?
To start with we need to understand that pressure is not force. Force is pressure times the area the pressure is working against. Example 1 PSI against a 1 sq. in. area piston is only 1#s worth of force, but 1 PSI against a 2 sq. in. piston is 2#s of force. The pressure is the same, but the force is doubled. This works on cartridges as well, as they are really a piston trying to get out of a cylinder when fired. Please refer to the below picture of six common cartridges.
I have shown the cartridges with their approximate equivalent piston area below their name. I have also listed the SAAMI (from Quickload) maximum operating pressure (PMAX). There is also a SAAMI MAP (Maximum Average Pressure) which is usually 5000 psi less or more. You will notice that the four left hand ones are AR 15 “compatible” cartridges. These will work in the AR 15 as long as adjustments are made to allow them to fit. The two to the right will work in bolt actions, and larger pattern semi-automatics. Please notice that the 5.56, 6.5 Creedmoor, and the 30-06 have similar working pressures, and that the 224 Val., 6.5 Grendel, and 7.62×39 have lower working pressures then the 5.56 despite the fact that they can be run in the same AR 15 platform.
Hopefully you either know the reason why the 224V, 6.5G, and 7.62×39 (magic three) are pressure de-rated, or willing to find out.
In the case of the AR 15 the locking area of the bolt is fixed (7 narrow bolt lugs against 7 barrel extension lugs) and there is no way to increase the area as cartridge rim dimensions increase. When the cartridge rim diameter increases, the bolt strength decreases, and the cartridge thrust also increases due to the increase in equivalent piston diameter. So now we are going to do a little calculation to demonstrate the effect of larger diameter cartridges. I will deal with 5.56 and 7.62×39 here as I have result pictures later on in the article. You can calculate 224V and 6.5G for yourself.
- 5.56 at full allowable pressure. 62366 psi x .11223si = 6996# of force. (I know you will say that much force will kill me, but the force is only being applied for 1 milli-second or less. So, you feel only 7# of force plus the gas blast released as the bullet leaves the barrel. There is weapon inertia to overcome as well.)
- 7.62×39 at full allowable pressure. 51488 psi x .15275si = 7865# of force. (But that is more thrust that 5.56, how can that be?)
- At what pressure does 7.62×39 equal the bolt thrust of 5.56? 6996# / .15275 = 45800psi. (That seems to be the similar to that listed in USSR documents and SAAMI MAP (Maximum Average Pressure.).
- Just for comparison 6.5 Creedmoor thrust is 63091 psi x .17572si = 11086 psi. (Oh, gee I need a stronger bolt or a bigger gun.)
So now we understand the pressure to thrust relationship, lets take a look at the AR 15 bolts. It should be plain from this view that the 7.62×39 bolt has less material in the recess face area. Even though the material is the same as the 5.56 bolt, there is less of it, and it is thinner in critical areas. As the bolt is thrust rearward there is about .0350” less thickness in the web between the lugs. This allows the lugs to rotate toward the center of the face during firing, and this flex fatigues the metal until it breaks (this one broke, and the two adjacent lugs are cracked). Actually, the AR bolt is a very compromised design as the lugs overlap the bolt recess, unlike most bolt actions where the lugs are completely behind the recess. What saves us is, Jim Sullivan designed in enough material to be safe at 5.56 case head diameters when he converted the AR 10 to 5.56. If Sullivan had to make an AR for 7.62×39, he most likely would have made the bolt larger in diameter, somewhere between AR 15 and AR 10. This problem, and mag well width, is why you will not see an AR 15 adopted into military service in any of the magic three, or 6.8 SPC. My opinion anyways. Slovenia adopted 6.5 Grendel, but not in the AR platform.
In this sideways view I have tried to show how limited the material widths are in the lug area. I think you can see that there are two basic degradations of strength in any AR 15 bolts that are made to accommodate a bolt face larger than required for 5.56. 1 increased thrust, and less material to resist the thrust. Next, we will discuss what the failed bolt owner did wrong even though he thought he was following good practice and reading the pressure signs properly.
Process Analysis: By Failure
- Not understanding the physics involved and the relationship between pressure and thrust.
- Using techniques that mostly work for cartridges that have operating pressures in the 60000 psi range for those that have operating pressures in the 50000 psi range.
- Being tickled pink that you are getting higher velocities out of his 16” barrel than the factory (with pressure equipment) get with a 20” barrel.
- Thinking that a load manual will keep you save under all circumstances.
Commentary: as outlined above.
- Sorry to say, I seem to run into lots of reloaders that skipped Physics in high school, or teachers that were not passionate on the subject. Basic physical relationship knowledge goes a long way for keeping out of trouble. Reloading is not a rote skill, it is a participation skill with satisfaction benefits. I am afraid that some of the modern teaching methods and curriculum leaves a lot to be desired.
- (A.) I see many reloaders on YouTube using the same pressure detecting techniques, that work on high pressure cartridges, for medium pressure cartridges. Classic example is swapping to a harder primer meant for 5.56 on their 6.5 Grendel because he is getting cratering or flattening. If you are getting craters with your Rem 7 ½, I would look hard for a reason before substituting a CCI 41. In the case of small pistol primers, there is a reason Remington (I am not shilling for Remington) makes size 1 ½ and 5 ½ Small Pistol Primers. The priming compound is the same in both (Remington Tech Support), but the cup is thicker in the 5 ½. Put a 1 ½ in a 9mm Luger, and it will flatten a lot. You can use a 5 ½ in a 38 Special, but it will not flatten until you reach dangerous pressure, but may misfire if your hammer fall is light. If you see ejector swipes on your non-5.56 cartridges you are likely over pressure. 2. (B.) Everyone wants to meet, or exceed, factory velocity, but we do not have the same powders the factories do. The factories can get any blend they desire, while reloaders have to be satisfied with tiered canister powders (some canister is the same as factory). Yes, sometimes you can exceed factory safely, but it is rare. Lately, a lot of people are trying to match 224 Val. Factory published velocities, but the factories cannot even reach their published speeds without exceeding pressures. What is the evidence of this you say? Primer pocket expansion I say. If the factories are using different brass in their 224 Val cases than their 5.56 cases, I would be surprised. I do not have a hardness tester, but the file scrape test seems to show the brass is the same hardness by brand. If this is true, then what is the explanation why the 5.56 brass will reload 10 times, but the 224 Val. only reloads 2 – 3 times. My guess is that the slower powders stay at near peak pressure longer, thus exposing the brass to high pressure for a longer time.
- I think most reloaders have fallen under this spell. It seems so cool, and impresses some more than it should. I would be suspicious whenever it is claimed to be reached safely. Ask, me how I know, been there, done that. Enough said.
- Load manuals are produced by factories on their equipment, under their conditions. They are inconsistent in presentation. In that I mean, some leave out the brass manufacturer, or use brass that is not easily obtainable. Case in point in the Hodgdon 2018 manual for 7.62×39. They used Midway brass. How many have even seen Midway brass in that caliber. Midway does not make brass, they contract some manufacturer to place the Midway headstamp on it for them, or they contract for rebranding ammunition. Since you do not know who makes it you can not rely on its repeatability. In the case of this article, the reloader of the blown bolt used this Hodgdon data with Winchester brass. This brass weights about 131 grains, all my available sample of 7.62×39 brass weights about 118 grains. Case volume differences amount to about 2 grains of H2O. That according to Quickload means the pressure went from 48500psi to 56800 psi (using AA 2200 100% load density). This is a small capacity case and 2 grains means a big pressure change. Small cases get you in trouble faster.
Do not use high pressure techniques on low or medium pressure cartridges.
That is easier said than done because most rifle components are meant for high pressure cartridges. I tend towards the softer primers and if I get near to factory velocities I quit. I have a 264 LBC Savage 20″, and it has a bolt that will stand 60000 psi plus. If I stuff in enough powder I can reach 2700 fps with a 123 grain bullet without flattening CCI 41s or expanding the cartridge base. However, if I place that in my AR 15, I can break the bolt in 20 rounds. Ask me how I know. I do not load these of course, but tempted, as i can almost compete with the 6.5 Creedmoor guys. I now have a Creedmoor, temptation gone.
Please, most of your families still want you.
Comments are welcome.
I started reloading in 1968 while attending college and spent 4 years (high school) under the direction of a US Marine field armorer. Having retired recently, and now that I have time to ponder my hobby instead of concentrating on working for some company, I have assembled some of the things that I think I have learned over the years of playing around with firearms.