AR trigger question

[OliveDrab]

little history:

http://www.calguns.net/calgunforum/showthread.php?t=357847

-jp trigger springs
-polished trigger
-shortened pull (grip set screw mod)

ive shot about 500 rnds through it since (PMC BRONZE .223) functioned flawlessly. on my last trip out, i had 2 light primer strikes.

QUESTION:
if i reduce weight of the hammer will i get a harder strike on the firing pin or will i make the strike lighter?

i guess its a physics question.

any answers would be appreciated.

thanks
-OD

[Uncle Scary]

little history:

http://www.calguns.net/calgunforum/showthread.php?t=357847

-jp trigger springs
-polished trigger
-shortened pull (grip set screw mod)

ive shot about 500 rnds through it since (PMC BRONZE .223) functioned flawlessly. on my last trip out, i had 2 light primer strikes.

QUESTION:
if i reduce weight of the hammer will i get a harder strike on the firing pin or will i make the strike lighter?

i guess its a physics question.

any answers would be appreciated.

thanks
-OD

Reducing the weight of the hammer will make the hammer travel faster but not hit harder. I don't know if it will improve ignition.

I also installed a JP reduced power spring kit and polished the contact surfaces on my carbine's GI trigger. It improved the smoothness and weight of the trigger pull tremendously. I never had a failure to ignite with that setup, and I've shot both Wolf and Lake City M193, both of which are reputed to have hard primers.

[G-forceJunkie]

Good question. Will the additional velocity make up for the reduced mass? My SWAG is it will make it worse and would guess that the springs are wearing and softening up hence your problem.

[bjl333]

I am thinking for you to take the BCG apart to clean it !!! In a reduce tension kit the hammer is hitting softer. If the firing pin has more resistant inside then it won't go as fast against the primer.
Lastly check the primers of those 2 rounds and see if the primers are seated lower then everything else. Its a long shot but take a look anyways.

[Uncle Scary]

I am thinking for you to take the BCG apart to clean it !!! In a reduce tension kit the hammer is hitting softer. If the firing pin has more resistant inside then it won't go as fast against the primer.
Lastly check the primers of those 2 rounds and see if the primers are seated lower then everything else. Its a long shot but take a look anyways.

+1

Clean everything, make sure that the hammer isn't a tight fit on the hammer pin, apply grease.

[OliveDrab]

Good question. Will the additional velocity make up for the reduced mass? My SWAG is it will make it worse and would guess that the springs are wearing and softening up hence your problem.

the springs are about 2 months old, and 500 rnds though them.

I am thinking for you to take the BCG apart to clean it !!! In a reduce tension kit the hammer is hitting softer. If the firing pin has more resistant inside then it won't go as fast against the primer.
Lastly check the primers of those 2 rounds and see if the primers are seated lower then everything else. Its a long shot but take a look anyways.

the rifle was clean and lubed when i had the malfunctions... maybe 100 rnds max

+1
Clean everything, make sure that the hammer isn't a tight fit on the hammer pin, apply grease.

function check is good, everything is smooth.



seems as if the general consensus is that it was a fluke, not a major worry. ill keep an eye on it and be sure to save the rounds (if any) if they do not fire.

thanks!

[captbilly]

I though nobody would ever ask!

My degrees are in physics and thermodynamics and I have spent considerable time considering this exact question. Here is my somewhat educated take on the issue. If we don't consider the collision between the hammer and the firing pin then the answer would be that it makes no difference whatever if you change the mass of the hammer. Decreasing the mass will obviously make it move faster but the energy in the hammer is entirely dependent on the hammer spring. By using the reduced force spring set you have reduced the energy in the hammer when it strikes the firing pin, thats a fact (to be completely clear only the reduced strength hammer spring reduced the hammer energy, the reduced force trigger spring will have no effect on striking energy).

Now on to the question of hammer mass. Ideally the moment of inertia of the hammer is matched to the mass of the firing pin. There is some question in my mind whether the standard AR hammer is ideally matched to the firing pin mass but my measurements and rough calculations show that they are pretty well matched (within a few 10s of percent). If you reduce the mass of the hammer without reducing the mass of the firing pin then you are likely to reduce the energy transfered to the firing pin.

I have to go, I will continue later, after the lunar eclipse.

[Uncle Scary]

function check is good, everything is smooth.

I have the same trigger spring kit as you, and I also polished my contact points. I have not experienced any light strikes even on ammunition with hard primers. We can conclude that the problem may lie elsewhere. Maybe your AR is not going into battery. Check your buffer and buffer spring, perhaps?

[bombadillo]

I did the exact same thing that you are looking at doing which was polishing the sear surfaces, JP 3.5lb springs, and took the tail off of the single stage hammer essentially making it look like a JP speed hammer. It got light primer strikes on hard primered military ammo. I ended up just getting a Geissele SSA and that thing strikes so hard you could darn near break a finger in it. Honestly just sell your setup and get a cheap frank white trigger from compass lake and be done with it. He does the BEST bang for the buck trigger out there IMO. Great pull and really light weight for $109 without light primer strikes.

[captbilly]

My degrees are in physics and thermodynamics and I have spent considerable time considering this exact question. Here is my somewhat educated take on the issue. If we don't consider the collision between the hammer and the firing pin then the answer would be that it makes no difference whatever if you change the mass of the hammer. Decreasing the mass will obviously make it move faster but the energy in the hammer is entirely dependent on the hammer spring. By using the reduced force spring set you have reduced the energy in the hammer when it strikes the firing pin, thats a fact (to be completely clear only the reduced strength hammer spring reduced the hammer energy, the reduced force trigger spring will have no effect on striking energy).

Now on to the question of hammer mass. Ideally the moment of inertia of the hammer is matched to the mass of the firing pin. There is some question in my mind whether the standard AR hammer is ideally matched to the firing pin mass but my measurements and rough calculations show that they are pretty well matched (within a few 10s of percent). If you reduce the mass of the hammer without reducing the mass of the firing pin then you are likely to reduce the energy transfered to the firing pin.

I have to go, I will continue later, after the lunar eclipse.

What you really need to do, if you want to reduce the mass of the hammer without increasing the chance of a light strike misfire is to reduce the firing pin mass proportionately with the hammer mass. So if you are going to reduce the mass of the hammer by 1/2 then you also need to reduce the mass of the firing pin by 1/2, in order to keep the energy to the primer the same. You can buy a titanium firing pin from a number of sources for prices from $20 - $45. The titanium firing pins are about 40% lighter than a steel firing pin.

In your case, with the reduced force hammer spring, there is nothing that you can do to increase the energy to the primer short of replacing the hammer spring with a stronger one. I don't believe that lightening the hammer would help, nor do I believe that increasing the mass would help. It is possible that you could potentially gain some small improvement by either increasing or decreasing the hammer mass slightly but you would have no simple way of knowing which to do (lighter or heavier)

JARD makes a heavy hammer for AR-10 type rifles. In my opinion the reason why the heavy hammer is effective in reducing light strikes on AR-10/LR.308 type rifles is that the firing pins on those rifles is considerably heavier than the one on an AR-15. The AR-15 firing pin weighs about 7.5 grams while the AR-10/LR.308 firing pins weighs over 10 grams. Therefor the hammer mass for an ideal match to the AR-10 firing pin is about 50% higher than the one for an AR-15. This is, in my opinion, the reason why many AR-10/LR.308s are having light strikes, not simply because some 7.62 primers are harder (I don't actually even know if those primers are harder but I have no reason to doubt what others have said).

Disclaimer: I have not done anywhere near enough measuring, calculating or testing to know for sure whether I am really correct about any of this. I know that my physics is correct but I am not sure that I have modeled the firing pin to primer impact, and if my numbers are incorrect (the hammer is really way to heavy or way too light to match the firing pin, for example) then my conclusions would be way off as well.

By the way, unlike many upgraded triggers, the Geissele does not use a reduced force hammer spring. They use a two stage trigger and accurate geometry and smoother sear surfaces to get a smoother trigger. The Geissele also uses a much lighter hammer in order to reduce lock time. It would seem that by using the full force hammer spring that even with a substantially lighter hammer they still get sufficient energy to the primer, since I have not heard of any light strike issues with the Geissele. I certainly have not had any even on my LR.308.