AR Sponsor
Posted: 7/29/2015 10:27:34 AM EDT
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OK, I know that accidentally firing a .243 through a .308 barrel doesn't generally hurt anything...except maybe one's self-confidence a bit...LOL! I'd seen a few reports of this and other smaller caliber bullets fired through larger caliber barrels, and I'd read what the results usually turned out to be. In my case, however, I saw something a little different, so I thought I'd ask here.
Here's the story. Last year I bought a pair of Armalite AR10's in .243 and .308 with 20" barrels. In short order I could tell that I wasn't going to utilize the .243 very much, so I changed the barrel out to a 16" .308 to have a carbine length to compliment my rifle length .308. So, out shooting last Friday with both AR10's, I do the boo-boo of somehow picking up a magazine that was still loaded with .243. My 20" barrel .308 was shooting .5-1.0 MOA with PMC Bronze 147g FMJBT. I was both surprised and pleased to see my rifle shoot that well with bulk military ball. I switched ammo to see how some of my cheaper hunting loads would perform...in this case, Winchester 150g Powerpoint that actually turned out to be .243 80g Predator and Varmint soft point...they look identical except for caliber. I find recoil in my 20" AR10 to be super soft, so I didn't notice a difference in recoil except in reflection. I did experience an immediate shift of POI on the target at a consistent 5MOA to the 9 o'clock position with 5 shots looking through my spotting scope. I pulled the magazine and realized my error...crap! In my mind I remembered similar stories that indicated little chance for barrel damage in a case like this. I popped open the upper and checked the barrel...nothing unusual. I carefully fired a fresh .308 PMC Bronze, and it fired fine. I went back to the target, and the rifle immediately went back to .5-1.0 MOA. What a relief. I've since cleaned the rifle and had the barrel inspected by someone with a bore scope and experience. No visible damage or issue whatsoever. An interesting and informative lesson learned. In reading about incidents like these, I read that the spent cases should exhibit split necks. I reload but not for these AR10's at this point, so I'm used to inspecting cases. The 5 cases I fired had no split necks, but they were "belled out" to where they looked almost like spent .308 cases...logical I believe. The odd part is the primers. Each primer was pushed out uniformly the tiniest bit...maybe just shy of a millimeter. The firing pin crater was very normal and not mushroomed or otherwise deformed. My question is about the primer. I would think this incident would have yielded lower chamber pressure. To me the tiny amount of primer release would indicate high pressure. I'm sure I'm missing something simple here, but since I don't normally try to turn my .308 into a .243/.308 over-and-under rifle, I'm just curious about the .243 primers exhibiting what looks like to me a high pressure situation. Everyone knows not to intentionally fire the wrong ammo through the wrong firearm. So, just like the current stream of Direct TV adds that warn, "Don't be like that guy"...I warn, "Don't be like me"...LOL! |
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Primer back out is often an indicator of LOW pressure, not just high. Due to the case not being fully aginst the bolt face because of the firing pin pushing the case forward a hair (perfect headspace is not done in semis due to lower chambering force so there will always be a thousandths or two of headspace) at firing the primer backs out a bit. Normally the case moves back and re-seats the primer as the pressure builds. That is why cases stretch with repeated firings. Due to you extreme low pressure situation, the case did not move back against the bolt face and re-seat the primer to flush.
Look at any loaded, then fired case. When un-fired the primer will be .001-.003 below the case head, but after firing it will be flush. |
| Primers looked perfect except for the slight "back-out". What Big-Bore is describing now makes perfect sense to me. I had just never heard of a discussion about the primer issue in cases like this where it backed out. The "extra space" description seems to be the logical explanation. I'm glad I'd read about situations like this before it happened, or I'd have made the assumption that the barrel might be toast. Now, my situation of the .243/.308 issue may be one of the better scenarios when a mistake is made. Others can lead to damage and/or worse. After this incident I inserted a .308 cartridge into a .243 chamber out of curiosity, but it doesn't look like it will ever chamber enough for a firing pin to make contact or the bolt to close. Again, a good lesson without apparent consequences...this time. |
Nothing hurt but your pride this way. 
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