Will my 50BMG blow up?

A

alinwa

oft dis'd member
Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

Chamber diameter .7 in affected area

I'd like to be comfortable to 100,000psi

Lilja CM bbl
 
alinwa said:
Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

Chamber diameter .7 in affected area

I'd like to be comfortable to 100,000psi

Lilja CM bbl
Click to expand...

Are you going to use hankies for cleaning patches?
 
alinwa said:
Can anyone here validate my numbers on barrel size over the chamber on a 50BMG??

I recently blithely turned down the barrel of a 50BMG to 1.4 over a portion of the chamber, now I'm questioning my numbers simply because I can't find any evidence anyone's ever done this before. My numbers tell me that I'm safe clear down to 1.3 over the entire chamber but my math skills are barbaric.

Chamber diameter .7 in affected area

I'd like to be comfortable to 100,000psi

Lilja CM bbl
Click to expand...

I don't know the answer Al but as you know, peak pressure occurs some distance down the bbl. Those small pencil bbls on light hunting rifles in huge and very overbore cartridges make me both wonder and gives me confidence at the same time. Lol!
 
alinwa,

Check SAAMI specs, or the TM, since the 50BMG does NOT operate anywhere near 100,000 psi.....w/o a Bbl obstruction!

You're basically dealing w/ a 30-06 case enlarged/magnified X 2.
 
Last edited:
Dans40X said:
alinwa,

Check SAAMI specs, or the TM, since the 50BMG does NOT operate anywhere near 100,000 psi.....w/o a Bbl obstruction!

You're basically dealing w/ a 30-06 case enlarged/magnified X 2.
Click to expand...

I have no idea how I can use SAAMI specs nor "the TM" to answer my quite specific question.
 
alinwa said:
I have no idea how I can use SAAMI specs nor "the TM" to answer my quite specific question.
Click to expand...

You might check with Hodgdon about typical working pressures for .50 BMG loads. They may be able to answer your specific questions. Would any of the barrel makers have an idea.
 
antelopedundee said:
You might check with Hodgdon about typical working pressures for .50 BMG loads. They may be able to answer your specific questions. Would any of the barrel makers have an idea.
Click to expand...


I know perfectly well my working pressures.....stock doesn't interest me. I don't buy a 66 Nova to restore the L6 motor in front of a 2-speed powerglide.

Building a 50BMG like everybody else does it interests me slightly less than buying a crate of SKS's and some cases of steelcore ammo
 
ran this by one of my kids in church this morning, interesting take......

"since your chamber diameter=.700, your radius (working pressure) = .350" .... "And since your barrel diameter is 1.400 minus .700 the thickness is also equal to .350 therefore it's 1:1, ie chamber pressure = burst pressure"

So basically the tensile strength of the steel EQUALS the allowable pressure before she ruptures! And since Lilja uses a 4140 variant with a tensile strength of 95,000 it looks as though even a proof load should be OK (I'll probably be running this gun at 55-62,000psi)

But not the safety margin my initial figures indicated because I'd mistakenly used a figure of 120,000psi for my tensile strength.

I guess this one WON'T be a hotrod :) I'll stick with a full-sized shank for my "test to destruction" experimentation
 
The 'trick' to outrageously high chamber pressures is to have a 'built up' barrel and chamber area.
The outer layers are heater, the inner chilled, and then they are pressed together.
Think large gun artillery barrels.

You can often see the steps where the outer compressive layer ends.
If you can develop 50.000 to 100,000 PSI of compressive loads the inner metal has that much more 'room' in the load department.
Never seen in done on anything below .50 cal though.
 
Last edited:
bolt thrust

I'm sure you're aware but bolt thrust of a 50 BMG, at EQUAL PRESSURE is nearly double that of a ppc family cartridge, fwiw. I know you're burning a pile of powder, but pressure is pressure. Bolt thrust is a different ballgame with different case heads. Just saying, don't lose sight of that. No pun intended.:D
 
Bolt thrust in M2

We had many M2 machinegun barrel extensions break due to excessive bolt thrust. This was caused by soldiers lubing the ammo before firing. The brass was sticking in the chambers because it was softer than standard design. Some solider had the idea of lubing the ammo to stop the sticking. The soft brass came about from a second source ammo producer using a different case forming procedure, thus producing ammo with a softer case than the LC GI standard as it wasn't work hardened enough in the case forming process. This forced a brass hardness profile to be added to the TDP. Be aware of any older brass with an IMI or PMJ headstamp. It was also sold under other private party names. Use LC or WCC headstamp brass for consistent brass hardness.
 
How old is "older"?

MilGunsmith said:
We had many M2 machinegun barrel extensions break due to excessive bolt thrust. This was caused by soldiers lubing the ammo before firing. The brass was sticking in the chambers because it was softer than standard design. Some solider had the idea of lubing the ammo to stop the sticking. The soft brass came about from a second source ammo producer using a different case forming procedure, thus producing ammo with a softer case than the LC GI standard as it wasn't work hardened enough in the case forming process. This forced a brass hardness profile to be added to the TDP. Be aware of any older brass with an IMI or PMJ headstamp. It was also sold under other private party names. Use LC or WCC headstamp brass for consistent brass hardness.
Click to expand...

Sitting on a pile of new IMI brass that I bought perhaps 10-15 years ago to use when my LC brass was done...

GsT
 
GeneT said:
Sitting on a pile of new IMI brass that I bought perhaps 10-15 years ago to use when my LC brass was done...

GsT
Click to expand...
I was at that plant in 2008 looking at the problem. So brass produced before then up to around 2010 for the US Gov, stamped PMJ. They might have continued the process for civilian sales. It will only give problems with cycling in automatic weapon systems. We had no issues with sticking in Universal receiver or accuracy fixture.
 
Thank you for your thoughts re bolt thrust..... let me be clear.

I DO NOT intend to shoot these critters at BR pressures!!! I did run the barrel in question up to over (well over) 80,000psi but ONLY to "proof" it. I'm unwilling to waste strain gauges on this project, nor am I going to magnaflux or scan it...... I feel it's got MORE than adequate support based on my scaling figures.

Plus the shank portion of the barrel I'm asking about is less than 1/2 of the front of the case and in any case it's 20% larger in diameter than the barrel tenon.

ie THE THREADED BARREL TENON IS SMALLER than the front portion I'm asking about.

IMO this tenon sizing is on the action designer

And bolt thrust is also a design parameter, not something I can "support" nor "un-support"

Also, to be clear.....bolt thrust on the BMG is much more than double :) more like 3 and 1/2 times as much!

Yeahhhh..... all the parameters become almost counterintuitive when stuff gets BIG. To stick with my BIG FISHING analogy, the reason you've got to learn all new knots for 35lb drag pressure is that monofilament cuts itself off at just over 20lb no matter how thick it is.... and the superlines slice mono clean off at under 10lb.....so even just mono-to-mono a normal fishing knot in 20lb line will hold up just as well as 30lb, or 40lb or 80lb mono.
 
BTW, only a fraction of a percent of fishermen on this planet have ever experienced even 10lb drag pressure fighting a fish ;)
 
alinwa said:
Thank you for your thoughts re bolt thrust..... let me be clear.

I DO NOT intend to shoot these critters at BR pressures!!! I did run the barrel in question up to over (well over) 80,000psi but ONLY to "proof" it. I'm unwilling to waste strain gauges on this project, nor am I going to magnaflux or scan it...... I feel it's got MORE than adequate support based on my scaling figures.

Plus the shank portion of the barrel I'm asking about is less than 1/2 of the front of the case and in any case it's 20% larger in diameter than the barrel tenon.

ie THE THREADED BARREL TENON IS SMALLER than the front portion I'm asking about.

IMO this tenon sizing is on the action designer

And bolt thrust is also a design parameter, not something I can "support" nor "un-support"

Also, to be clear.....bolt thrust on the BMG is much more than double :) more like 3 and 1/2 times as much!

Yeahhhh..... all the parameters become almost counterintuitive when stuff gets BIG. To stick with my BIG FISHING analogy, the reason you've got to learn all new knots for 35lb drag pressure is that monofilament cuts itself off at just over 20lb no matter how thick it is.... and the superlines slice mono clean off at under 10lb.....so even just mono-to-mono a normal fishing knot in 20lb line will hold up just as well as 30lb, or 40lb or 80lb mono.
Click to expand...

You may have seen this but it's how to calculate bolt thrust along with flex and shear.
People wonder why a 6.5 Grendel AR15 has a lower saami max pressure than a 223/5.56. Well, there's exactly why and if you reverse engineer it, the math calculates to pretty close to exactly the difference in bolt thrust difference between the two case heads. IOW, the max Grendel pressure was adjusted down to give equal bolt thrust since the bolt lugs appear to be the weakest link in the AR15 platform. It's not so much the thinner area around the bolt lugs as it is the additional thrust, especially when people try to hotrod them, running the grendel up to 223 pressures, for example. These guns break lugs if they are run too hot. They didn't adjust pressures down due to a thinner barrel wall, it's the lugs. A good bolt gun is a different story but the thrust increase is the same. And you're right, the bmg has over triple the bolt thrust of a ppc case head. When I said over double, I was just basing that on the bmg being over twice as big around but ya can't do that.
Hope this link helps. Good info for your kind of project, nonetheless.
https://riflebarrels.com/a-look-at-bolt-lug-strength/
 
Last edited:
You must log in or register to reply here.
Back
Top