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Blow Forward Pressure

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golden24 View Drop Down
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    Posted: 18 Sep 2020 at 12:02am

Blow Forward Pressure

 

“Blow forward pressure is what damages the barrel latch in an out of battery firing. Look at the bottom of the extension lug at the opening for the barrel latch. You will  see raised metal at the back of the run out slot milled into the bottom of the barrel extension lug. This damage is caused by blow forward pressured engagement of the upper against the barrel latch. Usually the barrel latch lever will bend outward or snap off entirely.

 

This is why extension locking lugs setback and excessive wear is harmful and the reason I made a gauge to determine headspace and lug space so cartridges could be trimmed to a length to minimize lug space.

 

When firing in battery, If you have wear or setback on your extension lugs you will get blow forward of the upper to the limit of bolt lugs and if excessive can and will load stresses onto the barrel latch and cause barrel latch clocking and in extreme cases bend the barrel latch lever.

 

Always remember this. The recoiling parts have weight and the blow forward pressure is always greater than the driving force necessary to move the recoiling parts a given distance with enough energy to operate the accelerator and launch the bolt to the rearward cessation of movement necessary to complete a loading cycle.  Blow forward pressure very real and is nothing to underestimate.

 

The damage I have seen in these out of battery instances are pretty common, usually the recoil rods are bent, the cocking piece and or ears on the back of the bolt are damaged, the bolt ring is bent rearward and the rotation pin hole is elongated, the rotation pin is bent or cracked, the recoil springs are damaged from over compression and sometimes the firing pin is bent.

 

When the bolt is dropped on the older Auto Mag pistols the frontal area of the bolt lugs crash into the back of the barrel. This is not good as it causes an inertial launch of the firing pin while the bolt is bouncing rearward and unable to lock up. The higher the forward velocity of the bolt the greater the chance of an inertial launch of the firing pin delivering enough energy to detonate a primer, this why generally the out of battery occurrence happens when you point the pistol downward and drop a round in the chamber then release the hold open by depressing the safety lever or loading the last round out of the magazine while firing. Loading the last round out of the magazine you do not have the deceleration of the bolt supplied by pressure of the subsequent round in the magazine bearing against the bottom of the bolt when the bolt is moving forward.

 

The earliest firing pin print from the old Auto Mag company indicated a 65 grain firing pin which was later changed to a 75 grain firing pin with a notation “to improve reliability”. I was told by one of the old engineers who was aware of the change that it was thought the heavier firing pin would be more resistant to inertial launch. Nothing could be further from the truth. After doing the math and taking into the consideration the necessary pressure adequate to compress the firing pin spring and have enough firing pin protrusion through the breach face with enough energy to detonate a myriad of commercially available primers that a firing pin of a weight of 55 grains was optimal. The firing pin was thoroughly tested and I have been unable to duplicate an out of battery occurrence or even put a mark on a primer even while doing a drop test from 10 feet onto plywood. The way this test is performed a tube is attached to the top of the upper assembly and a cable through the tube attached to a spar on top of a 12 foot step ladder and the lower end of the cable secured to the plywood. The pistol is held to a measured height of 10 feet and dropped. The cable/tube method assures a hard hit on the muzzle which will immediately set off a primer with a firing pin weighing 75 grains but will not mark the primer at all with the 55 grain firing pin.

 

My suggestion to you guy’s shooting the older pistols would be to replace your steel firing pin with the titanium firing pin along with the firing pin spring and new inner and outer hammer springs.  Our new firing pin springs are easy to identify as they are closed end and flat ground in case you get them mixed up.

 

If the bolt lugs are not damaged it was an out of battery occurrence. If the bolt lugs are damaged, cracked or sheared off then it was not an out of battery occurrence but a possible partial out of battery occurrence, over pressure round or metallurgical failure.

 

I have been told by some that passing my test findings on to the shooters are of no benefit. I beg to differ and directly question the intelligence of that individual. Incidentally I have never seen a bent rotation pin occurrence where the bolt rotation pin hole in the frame was not enlarged and elongated.

God bless

Tim”  (Tim Bell)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BEEMER1 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 12:52am
It makes perfect sense to me, thanks.

It describes perfectly the "parts gun" I posted pictures of.  I'll be getting some of the new firing pins before I fire any old AMP's.

The only other damaged barrel I have was with a "B" series solid bolt gun and I have always heard that the solid bolt guns were hard on barrel lugs.  Perhaps Tim will speak to that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Luvz2Shoot Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 1:48am
This is odd what Tim mentioned about the rotation pin.

Someone sent me a gun to look at because they couldn't get it to chamber a round without a huge exertion of force pulling back the cocking piece.

After a complete tear down the only things I noticed were that it had a chip out of hammer locking lip, and the bolt rotation pin was broken in half.

I replaced the hammer and bolt rotation pin.  Per the owner he agreed that I could put some rounds through it.  It functioned fine.

Before sending it back to the guy he changed his mind and wanted nothing to do with it.  We agreed on a price and I sent him the money and kept the gun.

To this day it is still one of my shooters.

I just looked at the rotation pin hole and it appears to be perfectly round still.  And the ring still looks good.

With these observations how else can the bolt rotation pin break?

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AGunNutToo Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 1:50am
Bingo.  Amen.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BEEMER1 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 2:16am
Originally posted by Luvz2Shoot Luvz2Shoot wrote:

This is odd what Tim mentioned about the rotation pin.

Someone sent me a gun to look at because they couldn't get it to chamber a round without a huge exertion of force pulling back the cocking piece.

After a complete tear down the only things I noticed were that it had a chip out of hammer locking lip, and the bolt rotation pin was broken in half.

I replaced the hammer and bolt rotation pin.  Per the owner he agreed that I could put some rounds through it.  It functioned fine.

Before sending it back to the guy he changed his mind and wanted nothing to do with it.  We agreed on a price and I sent him the money and kept the gun.

To this day it is still one of my shooters.

I just looked at the rotation pin hole and it appears to be perfectly round still.  And the ring still looks good.

With these observations how else can the bolt rotation pin break?

Thanks


Metallurgy problem?  Heat treat problem?  A guess.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Auto Mag Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 2:22am
Hummm,,,  Some posts just leave me scratching my head, wondering how we ran off the rails.

As for the term "Blow Forward Pressure"

I think a better description would be Newtons Laws of Motion.

Sorry still not buying this bolt drop slam fire issue as a "common" failure.  It may well be a legitimate problem, but clearly one of very limited occurrence. That said, it should NEVER happen, so truly understanding it is as important to me as any one. I just want to make sure I really know whats going on, not just what one person or the masses think is going on, and at this point one test is not sufficient for me to be satisfied or convinced.  Especially when I have not been able to duplicate the failure in any way with several test guns!

I think Tims original test; the basic notion of running the dummy round - bolt drop test is probably reasonable for a number of reasons, and kudos to Tim for creating this test, running this test, and for sharing his findings with us. Make no mistake, I applaud him for these efforts!

I also think replacing a 50 year old firing pin spring is also a reasonable upgrade any engineer or gunsmith would support.

But at the moment I would need something more tangible to convince me there is an absolute firing pin weight problem than what I have seen to date. If it is a 100% surety this is a problem why are you guys not all looking at your guns in pieces, or at least more of you than than the single event we saw in the video which was the first in the history of this website and all its inclusive posts and the countless thousands of rounds you folks have successfully fired through your Auto Mags.

As for the 10 foot drop test, sorry, if any of you are dropping your Auto Mag 10 feet onto a hard surface you are likely going to see several problems and certainly a discharge chief among them!  But since the gun would be in battery, it should not bend the ring, break the rotation pin or cause similar out of battery problems.

Sorry I have no idea why any gun would ever be expected to suffer a 10 foot drop onto a hard surface without a discharge or why you would create such a test.  Quite frankly if this actually happened, and you dropped your gun from 10 feet onto a hard surface, your concern should be who will get shot, not damage to your gun!

I seriously doubt that the New Auto Mag company will be including any statement in their sales literature that the New Auto Mag is guaranteed to be drop-proof to 10 feet, this whole notion is absurd and honesty detracts from a potential legitimate problem, and it does nothing to assure us that a firing pin change may or may not fix the problem should there be one.  In addition to all that, the tests I watched were not even on original Auto Mags, so at this point we dont know if the potential problem is unique to the new guns or new top ends when they use original AM firing pins and springs.

Sorry, I am not looking to rain tears of doubt on the new firing pin parade, but I am also not one to buy snake oil every time a new bottle is brewed up.

I doubt it will harm your guns to install the new firing pin, but thats not whats in question. 

The concern raised was, is there a slam fire problem in original Auto Mags, then does it present itself in all guns, and with all brass, and then what is the cause. And once we can be assured of all that, then a fix can be determined.  And if that fix is a replacement lighter firing pin, I can assure you I will be the strongest proponent of it!

If any of you are convinced this is a universal problem and that the new firing pin is the absolute fix, I am not in any way advocating you not follow you gut instinct and do what you feel is right and necessary.

My path is taking me in a different direction that required more validation.

In the end, I hope none of us ever suffers slam fire.

ATB

GH


Who was that masked man,,,
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Auto Mag Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 2:31am
Originally posted by Luvz2Shoot Luvz2Shoot wrote:

This is odd what Tim mentioned about the rotation pin.

Someone sent me a gun to look at because they couldn't get it to chamber a round without a huge exertion of force pulling back the cocking piece.

After a complete tear down the only things I noticed were that it had a chip out of hammer locking lip, and the bolt rotation pin was broken in half.

I replaced the hammer and bolt rotation pin.  Per the owner he agreed that I could put some rounds through it.  It functioned fine.

Before sending it back to the guy he changed his mind and wanted nothing to do with it.  We agreed on a price and I sent him the money and kept the gun.

To this day it is still one of my shooters.

I just looked at the rotation pin hole and it appears to be perfectly round still.  And the ring still looks good.

With these observations how else can the bolt rotation pin break?

Thanks


I would speculate several potential ways the pin would break.

First look at the pin, think about putting it in the ring on the frame, now think about hitting it with a hammer hard, like the force  of a 44 magnum round driving an Auto Mag bolt back and contacting the pin. It will fail instantly!

So why would the bolt hit the pin???

Poorly fitted gun.

A defective pin.

An overloaded round.

A hollow point jacket separation, with the jacket stuck in the barrel.

Other reasons unknown.

There is certainly not one single answer for this.
Who was that masked man,,,
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Luvz2Shoot Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 3:09am
I honestly didn't think the bolt hit the pin on the rearward motion.  I though it stopped just shy of hitting it.

I guess there could be a bunch of reasons.  

Thanks for the reality check.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Auto Mag Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 3:22am
Originally posted by Luvz2Shoot Luvz2Shoot wrote:

I honestly didn't think the bolt hit the pin on the rearward motion.  I though it stopped just shy of hitting it.


It shouldnt hit the pin in an ideal situation, but things usually break because something went wrong.
Who was that masked man,,,
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Luvz2Shoot Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 2020 at 4:33pm
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