off center primer strikes

[tonelar]

So, I brought my newly finished Group Industries pistol to the range today and it does this to me.

From the three attempts, it appears the barrel is sitting low in the receiver. Strike is occurring consistently at 12 o'clock.

DRATS

I loaded the same rounds in my other 9mm handgun and they touched off fine. Anyways, I'm guessing that if I move the barrel up in the receiver by between .001" and .002" this should fix the FTFire.

My question for y'all is: How close to center do primers need to be hit?

[ke6guj]

or the top cover is too high letting the bolt sit high

[tonelar]

first thing I checked, and with .006 feeler gauge in it won't move with ease

[ke6guj]

"newly finished" as in you put your parts into a factory-welded GI or Vector-finished GI shell? Or you welded up a GI shell with a a weld kit?

if the latter, does it have a semi-auto feed ramp with the ring? if so, not much you can do to move the position of the breech end of the barrel.

if it has a non-ringed feedramp then moving the barrel position is easier. either need to cut the welds to the trunnion, reposition and reweld, or slice the receiver and then reweld.

a picture down the barrel should show how far off-center the firing pin is.


OR, the bolt had the firing pin drilled off-center and swapping bolts could fix the problem. if this is an SA bolt made from a converted FA bolt as opposed to an OEM SA bolt.

[tonelar]

newly finished as in bought from someone who had completed the weldup, but not gotten a bolt nor striker yet.

so, I'm going to measure this compared to my Vector mini and figure where to go from there. Hopefully his trunion welds aren't that hard to grind out.

[ke6guj]

you should be able to drop in the mini bolt to see if the firing pin is in the same spot down the barrel. just to eliminate a bad bolt as the issue

but if he didn't have a bolt before he did the weldup, I'd guess that the barrel is just a little off. it doesn't take much.

[tonelar]

Tried the Vector bolt with an empty shell... same point of impact. Comparing the two, this one's geometry is off by under .003"

I'll see how far I can get tomorrow at the shop.

PS this one is also a mini.

[ar15barrels]

My question for y'all is: How close to center do primers need to be hit?

The closer the better.

Look at this cutaway view of a primer:



The primer works by the firing pin crushing the priming mixture between the anvil and the firing pin.
The further from center that the firing pin strikes, the less of the charge that is actually getting crushed against the anvil.

Here is a picture of a fired primer in the case:



Notice how if the firing pin hit is way off center that is is not even going to crush the mixture against the anvil at all?

[tonelar]

Thanks Randall.

I'm going to try for dead center (of course). But, first, I'll see how close I can get by moving the feedramp / restrictor ring upwards in the receiver.