I came upon this 2008 thread today and read an interesting discussion:
https://thefiringline.com/forums/showthread.php?t=317246
I was going to reply but realized it's probably too old so I saved what I wrote and I am putting it here. this is just an old gun builder's opinion and I am lucky to be able to spell physics.
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I have read what Kraigwy and Snuffy (both Senior members) have said about bolt opening. Three times.
I will not take a side. I will just tell you mine.
In Vietnam my address was 460th Atry (AW SP). that AW stands for automatic weapons - SP is for self propelled. On Ben Het Firebase we had it all - Quads, Dusters (twin 40's). Even with all we had, the only gas operated guns we had were the 16's and the M60's.
Down through the last 50 years I have owned a number of "gas guns". Always have had two Garands (one a less than 200m serial #), one M1A, and my 1970Grade 2 BAR 30/06.
To explain any gun expertise I've gleaned through these years I built 1911's for 25+ years, build true line bored Shilen barreled Ruger custom cylindered Bisleys, and was a precision tool & die maker for 30+ years.
These 1911's are not gas guns but I believe I do understand them and they have a bearing here as an example.
All talked (or argued) about above, whether it's the explosion of the powder, the transference of the pressure to the op rod, etc happens in a pulse moment.
It's how we figure the best powders to fit in this pulse moment parameter and have them operate to their best effieciency and safety.
I'm going to delve into trouble here but in a way, the 1911 and this BAR are identical in the aspect we are talking about. Pressure.
The gas gun pulse can be likened to what's called barrel timing or dwell in the 1911. It's right there is one of Mr Browning's original proto's. When the big light hits the back of the bullet that slide starts rearward. It's spring loaded and I can assure you it does at that instant. In a well "timed" gun, the two barrel feet "ride" on the slide pin for .100 before the link pulls the barrel down out of lug lock with the slide. That .100 gives the bullet time to exit the muzzle and most importantly pressure to drop away, the case springs back from being swelled to the chamber sides and barrel/slide lugs un-weld themselves, the case releases and is extracted.
If this barrel feet .100 "ride" is shorter or not present, it's ugly. At the least the shooter is met with gas blast to a degree and case extractor rims are bent or at extreme, torn. At the least, he has a filthy port/chamber area gun after only 10 rounds. And we are dealing with only 19-20m psi in a 1911.
Apply this same thing to the BAR gas gun (at 50m psi). If that bolt started any rearward movement trying to pull the case before the bullet exited the barrel and pressure dropped, every round we shot would have a piece of brass torn from the case rim that matched the extractor face surface. I believe any gas gun would have this happen.
It just couldn't keep from it. That physics thing.
Will
'69-'70
https://thefiringline.com/forums/showthread.php?t=317246
I was going to reply but realized it's probably too old so I saved what I wrote and I am putting it here. this is just an old gun builder's opinion and I am lucky to be able to spell physics.
--------------------------------------
I have read what Kraigwy and Snuffy (both Senior members) have said about bolt opening. Three times.
I will not take a side. I will just tell you mine.
In Vietnam my address was 460th Atry (AW SP). that AW stands for automatic weapons - SP is for self propelled. On Ben Het Firebase we had it all - Quads, Dusters (twin 40's). Even with all we had, the only gas operated guns we had were the 16's and the M60's.
Down through the last 50 years I have owned a number of "gas guns". Always have had two Garands (one a less than 200m serial #), one M1A, and my 1970Grade 2 BAR 30/06.
To explain any gun expertise I've gleaned through these years I built 1911's for 25+ years, build true line bored Shilen barreled Ruger custom cylindered Bisleys, and was a precision tool & die maker for 30+ years.
These 1911's are not gas guns but I believe I do understand them and they have a bearing here as an example.
All talked (or argued) about above, whether it's the explosion of the powder, the transference of the pressure to the op rod, etc happens in a pulse moment.
It's how we figure the best powders to fit in this pulse moment parameter and have them operate to their best effieciency and safety.
I'm going to delve into trouble here but in a way, the 1911 and this BAR are identical in the aspect we are talking about. Pressure.
The gas gun pulse can be likened to what's called barrel timing or dwell in the 1911. It's right there is one of Mr Browning's original proto's. When the big light hits the back of the bullet that slide starts rearward. It's spring loaded and I can assure you it does at that instant. In a well "timed" gun, the two barrel feet "ride" on the slide pin for .100 before the link pulls the barrel down out of lug lock with the slide. That .100 gives the bullet time to exit the muzzle and most importantly pressure to drop away, the case springs back from being swelled to the chamber sides and barrel/slide lugs un-weld themselves, the case releases and is extracted.
If this barrel feet .100 "ride" is shorter or not present, it's ugly. At the least the shooter is met with gas blast to a degree and case extractor rims are bent or at extreme, torn. At the least, he has a filthy port/chamber area gun after only 10 rounds. And we are dealing with only 19-20m psi in a 1911.
Apply this same thing to the BAR gas gun (at 50m psi). If that bolt started any rearward movement trying to pull the case before the bullet exited the barrel and pressure dropped, every round we shot would have a piece of brass torn from the case rim that matched the extractor face surface. I believe any gas gun would have this happen.
It just couldn't keep from it. That physics thing.
Will
'69-'70