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Posted: 12/13/2004 6:54:49 AM EDT
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I have an SPR upper Im playing with that is turning out to be VERY accurate however it will occasional short stroke. Since I'm still swaping components and debating KG'ing the whole rifle. I havent done my usual excessive use of red lock tight rountine. I have had 3 pierced primers (using handloads - that are meticulously loaded, ie. neck turned, same weight cases, flash-holes deburred, properly trimmed OAL, primer pockets trued, and no, none of the primers are seated too high). The gas block has shot itself loose and migrated forward twice when I noticed short stroking. The other time it remained tight. When I have pulled the gas block, the burn mark from the gas block is correctly located around the gas port. I suspect the short stroking to be related to gas block alignment problems - I was initially unsure where to leave a gap behind the gas block or butt it up against the barrel step. the answer was solved with a caliper - Yes. However, I was wondering can a pierced primer cause short stroking? Ive had had a few pierced primers in different weapons over the years, however have never noticed it causing a cycling problem (at least that I can recall). I figured someone here will know the answer for sure. Specs of the upper. 1. WOA 1:7 18 in SPR barrel, rifle length gas system, matched bolt, standard ar-15 weight carrier 2. PRI gas block. 3. PRI rifle FF tube 4. CMT upper, M4 feedramps, reciever face trued and threads checked for square. 5. Bushmaster lower, armalite match trigger (Jard copy) 6. LMT stock with H buffer, buffer spring has 100 rounds fired over it. 7. Standard AR-15 firing pin. Side note. Firing pin protruded within spec, but has been replaced since the tip was cratered. |
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Regarding pierced primers, Conditions that would lead to this may be firing pin beyond the .038 protrusion, the wrong primer used (such as pistol instead of rifle), or the round loaded too hot. Now if the first two on conditions on your rifle/loads are correct, then your left with loads that a beyond the envelope of the rifle, and the primer is plasma flowing at ignition (due to the too high of working pressure). Normally, load lots that are too hot will have the tell tale signs of flattened primers through out the lot, or just bind/pressure stick in the chamber. But, if your not getting full lot signs, and the piercing are sporadic, then it may just be that some of your loads are too hot, and the short stroking is due to chamber bound cases on these select few rounds. Time to take a step back and reevaluate your reloading method, Some powders flow very uniform threw powder dispensers, while others do not. If you running a progressive press with the wrong gunpowder (will not meter), you may be under loading some rounds, and over loading others. Bottom line is to throw at least 10 charges threw the dispenser and weights them all. Once you have confirmed that they are all uniform, then it time to lock the slide down and start reloading. Also, make sure that your reloading bench is rock solid, and the press is not being dance on the pulls. Long extruded powders have a tendency to cling to any little bur in the dispenser, and with this, a few grains of powder may not be dropped, and carried over into the next case. Added: Make sure that all of your cases are trimmed to the correct length. Cases that are beyond the Max allowed length may be wedging the bullet/case beyond the last of chamber cut, and this alone with cause the round to act as if it was loaded too hot. As for long loading, my guess is that your still within the Mag limit length, and your not getting bullet embedding of the rifling at loading. |
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All cases are full length resized, checked with a RCBS precision mic to verify that they are .003 inches under sammi min (ie, I push the shoulder back .004 in. in this case as the fired cases measure .001, at the shoulder, over min). Cases are trimmed to 1.750 OAL after resizing. Im using Varget, at 25 grains and a 69 gr sierra HPBT, moly coated, velocity is 2612 on average with an ES of 14.6. All powder charges are hand weighed (digital scale), though my redding powder measure throws charges (with extruded powder) with in .1 grain. The bullets are seated to 0.013 over sammi Min, which in my match rifle allows them to be 0.003 inches into the rifling. In this rifle, the 18'' SPR clone, with its wylde chamber they are no where close to touching the rifling. Additionally, the primers on the non-pierced rounds show no signs of excessive pressure. I wont hazard a guess on those that are pierced since my observations regarding over pressure come from examining non pierced primers. The old firing pin protruded .031 inches, one closer inspection the tip is 'sharper' or more needle like than the replacement, but Im not sure I can mic it accurately since it is tapered. So, Will/can a pierced primer cause short stroking? |
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If related to over pressure, yes. If related to firing pin simple punch threw (small), The rifle has to be on the borderline of short stroking (i.e., tight, upper bearing surface binding) from the start. I have blown a few primers related to using pistol primers (not paying attention and just garbing shit off the shelf). On these, there was lots of smoke threw different rifles, but still they all cycled. Now if your punching/blowing big holes in the primer, that is a completely different story. My guess is that your loosing at least 15,000 PSI, which will reduce the pressure being sent down the gas tube for unlock. Also, on the old firing pin, you can save it instead of trashing it. Chances are you only need to take .004 off the tip when you clean it up, and you can set back the stop collar to get it back to the .032 new range. Added: Just a thought, are all the pierced primers the same case lot (being that a different case type than the reast that may have a reduced case volume). |
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Yes, All the cases are from the same lot LC 85. Stems back to my college days when I had more time than $ so I sorted through several thousand until I had cases that weighed within 1.0 or 1.5 grain range (I cant remember since its been a few years). Then I spent long hours trying to make them perfect. I think I actually checked at the time to verify that same weight was reasonably equivalent to same volume, but dont quote me. I will say that this brass will be sorely missed when it finally dies as it shoots as good or better than anything else Ive ever shot and I damn sure dont have the time to go thru such a selection/cull and prep process again - LOL, I should have worked more and bought one of those 2K 95+% thompsons instead of playing with brass. Thanks for the tip on the firing pin - I'll save it. As to the size in the primers - they are very small. I havent measured but they look smaller relatively than some of the blown 308 cases I keep to teach new reloaders with. |
Dude, you’re not alone. I still have crates and crates of LC Match brass (30-06 and 308) from my youth of HP shooting. Add to what you have done for a single rifle, and multiply it a couple of times for different rigs, including the time spent fire forming. Hell, I don't want to even think about all the time I spent primer pocket/flash hole unifying, neck turning, and case trimming alone, much less load testing for the numerous barrel/rigs that have been shot out. Add that to chasing perfection with rebuilding rigs to make then shoot tighter and the mind just starts to spin. Funny part is I just finish a new NM Garand a while back, and was testing it at the range using some of this brass. Trust me, I was like a chicken hawk over fresh kill on every spent case ejected. At one point, I even hid the range broom to keep the range officer from even thinking about sweeping up my brass. |
I remember it got to the point that my then girlfriend (now wife) wouldnt go with me to the range because I was so concerned about losing prepped brass. Stupidly, I once even said to her that her job was not to talk to me it was to watch where the rifle threw it so it wouldnt get lost - that pretty much caused a 5+ year hiatus on her attendance. NOTE to fellow shooters: Dont be this stupid  - If you are she'll learn to HATE your guns and the time you spend shooting. Deprogramming takes a lot of time and pain.When I hung my shooting coat up and moved into IDPA/IPSC she was shocked when I said, "Honey, you dont need to pick up anymore cases. We have enough." Big suprise, now she'll actually come to the range and hang out. Of course, with 3 kids shagging brass is no longer the hassle it used to be. 
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The having kids part is nice. I put my two boys to work down in the gunroom when they were around 13 reloading ammo. Figured that if they were going to shoot in, then should reload it as well. Now the little shits are off at college, and I am once again forced to load my own. God I miss the days of child slave labor.  |
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