Gunsmithing Question - 50BMG Chamber Cut

[reuben.goldberg]

I recently acquired and refurbished a rather dated State Arms Gun Co. 50BMG built by the legendary Klaus Horstkamp. Upon inspection of the chamber, it is apparent that the rifling lands are intentionally cut starting approximately 0.250" beyond the case rim lip. In other words, there is at least a 0.250" jump before the bullet engages the rifling. In fact, if one would load the cartridge so that bullet ogive actually jammed the lands, the overall cartridge length would be approximately 6.00", a whopping 0.550" longer than 5.450" spec.

Questions: Is this typical for cutting the 50BMG chamber? If not, what would Klaus have been thinking in creating this jump? And lastly, given I'm fairly new to reloading the 50BMG, how would you recommend I set the overall cartridge length for maximum potential?

Your thoughts are appreciated.

 

[jonaddis84]

How are you getting the length where the bullet will touch the lands? Is that measured or a guess?

I ask, because all reamers have freebore, the area between the case neck and the start of rifling. And in that freebore is a 1 degree leade, so the bullet will begin to touch rifling before your freebore is up, its impossible to tell where the bullet will actually begin to touch rifling without knowing the exact geometry of the bullet and the depth of rifling, leade angle, etc...

Actually, .250" does not sound like very much on a BMG, considering you can have over 0.200" freebore on something like a 7RSAUM chambered for long action.

Ive never loaded BMG, so Im not sure whats feasible. But I would probably try to get a case that has very slight neck tension, just enough to hold the bullet in place, but be somewhat easy to slide also. Then pull the bullet way out beyond mag length. Chamber the dummy round and let the lands push the bullet in the case to where it would be just touching the lands. Do this a few times to get an average measurement. Then you have a starting point. If its longer than mag length, just load mag length and go.

 

[reuben.goldberg]

How are you getting the length where the bullet will touch the lands? Is that measured or a guess?

Ive never loaded BMG, so Im not sure whats feasible. But I would probably try to get a case that has very slight neck tension, just enough to hold the bullet in place, but be somewhat easy to slide also. Then pull the bullet way out beyond mag length. Chamber the dummy round and let the lands push the bullet in the case to where it would be just touching the lands. Do this a few times to get an average measurement. Then you have a starting point. If its longer than mag length, just load mag length and go.

Actually, that is exactly what I did. I built a dummy round with a bullet gripped finger tight, and then chambered the round until the .51 caliber bullet ogive just engaged the lands. So I am quite confident as to the maximum OCL vs. spec 5.450" across the limited variety of .51 caliber bullets available. The rifle is a single shot shell holder bolt, by the way.

I've loaded a handful of test rounds with the OCL set just 0.05" short of the lands (easily chambered), and the groups are measurably tighter.

So from what I gather this freebore "jump" is not unusual, especially in larger calibers. Yes? But this 0.250+" seems excessive, even tho it must have been intentional. As a person accustomed to jamming the lands, with the .308 for instance, running out over 1/2" to do it with this particular rifle barrel seems a stretch. It just makes me wonder what someone as finicky as ol' Klaus had in mind.

Any 50BMG shooters out there with similar freebore in their chamber cuts?

 

[jonaddis84]

Ah gotcha, now knowing it's a single shot, he probably did that so you can get more case capacity thus more velocity before pressure.

Same idea as the 7 saum analogy. A short action reamer only has about .070" freebore where the long action has .188 minimum to take advantage of more case capacity with longer bullets.

Galaxy S3 on tapatalk

 

[reuben.goldberg]

Ah gotcha, now knowing it's a single shot, he probably did that so you can get more case capacity thus more velocity before pressure.

Same idea as the 7 saum analogy. A short action reamer only has about .070" freebore where the long action has .188 minimum to take advantage of more case capacity with longer bullets.

Galaxy S3 on tapatalk

Thanks, jonaddis, you're probably right. Using H-50BMG powder, a case will accept only some 250 grs without moving those long-ass heavy brass solids forward. But it'll take a better man than me to shoot 250+ grs of H in the thing. Yikes.

 

[rjacobs]

I guess I never realized the .50bmg holds 250gr of powder. I now want one.