Why are gas gun more sensitive to firing pin size vs bolt gun?

[Caterpillardoc]

I put a Aero 6.5 Creedmoor AR 308 together and had problems with pierced primers until I installed a JP HP bolt and firing pin. The bolt carrier group I initially used had a standard bolt and firing pin. Zero problems with the rifle once the HP bolt was installed.

I also have a Rem 700 that I shoot the same ammo in with no problems. I have a friend who had the same experience.

Would someone please explain to me why gas guns with high pressure rounds are more sensitive to primer issues than bolt guns? I don’t understand what would cause this. It seems like the support for the primer and pressure would be the same regardless of platform. What am I overlooking?

 

[cal50]

Most semi auto gas guns use or has a swinging hammer and torsion spring powering it. It has less mechanical / inertia force holding the firing pin against the primer face than a firing pin and strong compression spring in your typical bolt rifle. Reducing the firing pin size (diameter) reduces the primer flow from less surface area for the pressure to push against and supporting the primer cup better.

 

[natdscott]

Most semi auto gas guns use or has a swinging hammer and torsion spring powering it. It has less mechanical / inertia force holding the firing pin against the primer face than a firing pin and strong compression spring in your typical bolt rifle. Reducing the firing pin size (diameter) reduces the primer flow from less surface area for the pressure to push against and supporting the primer cup better.

Damn. I never thought of that....

I HAVE hand-fitted a bolt and pin before, and it works great. I think I could blow a casing or break a bolt before that rifle would show any craters. That is also cautionary: not ALL bolts/pins will EVER show a crater, (and some always will) so don't use it as a pressure indicator on your loads.

 

[bfoosh006]

And... the AR starts unlocking the bolt within milli seconds.. so the FP / Bolt is not "fully" supporting the primer .

I wonder how long a AR FP is actually "in contact" with a primer ?

My large frame AR's all have Adj. GB's and heavy buffer systems... and I haven't found the "need" for small diameter FP's yet... I have always attributed that to the milliseconds longer "sluggish" cyclic speed ( from the lessened gas and heavy weights )

God knows if that is why.

 

[Caterpillardoc]

Most semi auto gas guns use or has a swinging hammer and torsion spring powering it. It has less mechanical / inertia force holding the firing pin against the primer face than a firing pin and strong compression spring in your typical bolt rifle. Reducing the firing pin size (diameter) reduces the primer flow from less surface area for the pressure to push against and supporting the primer cup better.

This explanation makes sense. The heavier firing pin spring on a bolt gun giving better support by keeping the firing pin in position.

I understand a gas gun starts unlocking quickly after firing, however the primer still remains supported by the bolt until ejection begins.

 

[cal50]

Bolt guns have a spring directly driving the firing pin to the primer.
Gas guns have a swinging hammer that has to travel an arc , hits a separate firing pin that is then driven to the primer.
The mass and drag of the separate firing pin also reduces and slows the force of the hammer strike.

Bolt guns have an advantage in lock time and ignition force.

 

[ShooterwithNoName]

I had this problem recently on my 308 AR. I am already running the JP bolt and firing pin. I swapped the trigger out from a PNT to a RRA NM trigger. I also swapped a lighter hammer spring in along with the RRA set up.

First time out I fired 3 shots, fourth was a FTF. I dropped the mag and ejected the round and there wasn't even so much as a mark on the primer of the FTF cartridge. I checked my brass from the previous 3 shots and sure enough, round #3 had a pierced primer. The little "cup" that was punched out of the pierced primer had stuck to the tip of the firing pin and jammed the FP back into the bolt face so it wouldn't protrude.

So I ordered a heavy hammer spring and just got it installed a couple days ago. I had to tune the RRA hammer and disconnector interface a bit. The heavier hammer spring was not allowing the trigger to reset quite right. Polished the surfaces and dry fired it a bunch and now it's resetting like it should.

Going to try it out this weekend. Hopefully the heavier hammer spring will help prevent pierced primers. I have my gas system and buffer dialed in where I want them, so I'm hoping to not have to mess with them.

 

[cal50]

Pierced primers could be too much firing pin projection, sharp point vs radius or lack of support from the firing pin.
Light hits or misfires leans towards light or minimal firing pin force.
The heavier spring will help.

 

[FCS]

This explanation makes sense. The heavier firing pin spring on a bolt gun giving better support by keeping the firing pin in position.

I understand a gas gun starts unlocking quickly after firing, however the primer still remains supported by the bolt until ejection begins.

Actually, the primer isn't supported by the bolt. As soon a bolt starts moving the bolt face is pulled away slightly from the case head as there is clearance between the extractor and the rim. Once the case is broken free from the chamber by extractor, there is still pressure so case head gets slammed into bolt face.

Lotta stuff going in in a gas gun that just doesn't happen in a bolt gun since the case is extracted while still under pressure in the gas operated rifle.

 

[FCS]

I had this problem recently on my 308 AR. I am already running the JP bolt and firing pin. I swapped the trigger out from a PNT to a RRA NM trigger. I also swapped a lighter hammer spring in along with the RRA set up.

First time out I fired 3 shots, fourth was a FTF. I dropped the mag and ejected the round and there wasn't even so much as a mark on the primer of the FTF cartridge. I checked my brass from the previous 3 shots and sure enough, round #3 had a pierced primer. The little "cup" that was punched out of the pierced primer had stuck to the tip of the firing pin and jammed the FP back into the bolt face so it wouldn't protrude.

So I ordered a heavy hammer spring and just got it installed a couple days ago. I had to tune the RRA hammer and disconnector interface a bit. The heavier hammer spring was not allowing the trigger to reset quite right. Polished the surfaces and dry fired it a bunch and now it's resetting like it should.

Going to try it out this weekend. Hopefully the heavier hammer spring will help prevent pierced primers. I have my gas system and buffer dialed in where I want them, so I'm hoping to not have to mess with them.

Heavier hammer spring won't help.
If you are piercing primers with a smaller firing pin bolt, load is too hot and or primers are too soft.
You need a heavier buffer to delay unlocking, springs aren't going to do it.

Gas guns aren't bolt guns, run same load in a gas gun and you'll have issues. Try to get bolt gun performance from a gas gun, you'll have issues.

 

[cal50]

F=ma
(It definitely helps from an ignition standpoint)

 

[Caterpillardoc]

Cal50’s explanation with the different platform firing pin systems still makes the most sense to me. In a bolt gun the firing pin would stay in place easier by design and support the primer better than an AR.

In an AR the distance between the tip of the hammer and the hammer pivot pin would essential act as a lever and decrease the ability of the firing pin not to move.

I’m no expert, if I was I wouldn’t have posted the question, however I’m picturing in my mind the firing cycle in an AR. The bolt would support the primer until the bolt carrier group starts to move which would begin once the bullet gets to the gas port. The case base and bolt face would not be in full contact once extraction starts.

It would be interesting to see a pressure curve graph during bullet travel in a barrel for both platforms. Pressure at inches of bullet travel in the barrel.

I’m using the same 6.5 Creedmoor load in a factory 700 action rifle with Criterion barrel and Aero AR, 42 grains of H4350, 140 ELDM, Starline brass and CCI 450 primers. At any rate it shoots great in both rifles.

 

[ShooterwithNoName]

I assumed mine started piercing primers because the only change I made was the RRA trigger with lighter hammer spring. I ran a PNT Trigger before and I had never had an issue in approx 400rds. Within my first 3 shots with the RRA trigger I had the FTF and either shot 1 or 2 pierced the primer.

I was just shooting cheap PPU 168 match to try out the new trigger. Hopefully I'll have a chance to shoot it this coming weekend. Im interested to see if the heavier hammer spring helps. I'll run the PPU again, so everything is in line with my last trip. I usually only shoot FGMM from it or mild reloads.

I'll post what I find here, I'm curious whether the hammer spring is going to help now. I'll bring along a H3 buffer to try out in case the hammer spring doesn't help. Hopefully it will give you some good insight OP.

 

[Texascbr]

@ShooterwithNoName did you ever get results from changing the hammer spring?

 

[theLBC]

tangent topic is that i also see shake in my scope from the hammer just dry firing, so that has to affect accuracy i tiny bit.

 

[MontanaMan]

Heavier hammer spring won't help.
If you are piercing primers with a smaller firing pin bolt, load is too hot and or primers are too soft.
You need a heavier buffer to delay unlocking, springs aren't going to do it.

Gas guns aren't bolt guns, run same load in a gas gun and you'll have issues. Try to get bolt gun performance from a gas gun, you'll have issues.

This ^^^^^^^^^WSR's are not a good choice of primers, as they have a thin cup; Fed 205's & CCI 400's only get used with lighter loadings.

CCI 41's, BR-4, 450's, Rem 7 1/2's, Wolf Mag Match or the newer Fed AR primers are all fine with full power 5.56 loading................if you can ever find any of those ever again.

The primer supply situation is getting critical for any w/o a decent supply laid in.

MM