Gas port

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Gas port

Postby Siringo » Mon Jun 15, 2009 4:51 pm

Does anyone know what the gas port size is on the 16 inch barrel.
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Re: Gas port

Postby wildcatter » Mon Jun 15, 2009 6:37 pm

Siringo wrote:Does anyone know what the gas port size is on the 16 inch barrel.


From memory it's .093" It's the standard drill size that's closest to that .093 number, but I'm pretty sure that's it, I set the standard, but to be absolutely sure I'd have to look it up.

For you newer guys, a simple idea is to get a numbered drill set, you need them in any event, and start putting drills into the hole until you get the size correct..
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Re: Gas port

Postby Siringo » Mon Jun 15, 2009 7:31 pm

Wildcatter -- maybe this is a stretch, but the typical gas check on a lead bullet is .075" deep. With the gas port at .093", could it be possible that gas could get to the front side of the gas check and dislodge it -- ever so slightly. At a point in the travel of the bullet over the port, the gas check will straddle the hole, but it would not be sealed off. Gas would be acting on the base and travel around the gas check to the inside edge/groove. Maybe the reason why some are having such accuracy problems with lead bullets.

Your measurement has to be it. My divots in the sabot skirts measure the same.
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Re: Gas port

Postby wildcatter » Tue Jun 16, 2009 2:03 am

Siringo wrote:Wildcatter -- maybe this is a stretch, but the typical gas check on a lead bullet is .075" deep. With the gas port at .093", could it be possible that gas could get to the front side of the gas check and dislodge it -- ever so slightly. At a point in the travel of the bullet over the port, the gas check will straddle the hole, but it would not be sealed off. Gas would be acting on the base and travel around the gas check to the inside edge/groove. Maybe the reason why some are having such accuracy problems with lead bullets.

Your measurement has to be it. My divots in the sabot skirts measure the same.


Looked it up and yes .093 is correct.

Your idea is not a stretch at all and as lead bullets in gas guns is a relative rarity, the answer will most likely be something we have to Ferret out ourselves. This is a high-falutin' way of saying you have a clue and I don't, good thing we have at least two lead experts, how 'bout it BD and Boomer, watt-cha-thinks?
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Re: Gas port

Postby BD1 » Tue Jun 16, 2009 6:41 am

I wouldn't think that gas would get in front of the check as the port opens to the rear of the check as the boolit base passes the opening. I'd think the pressure would continue to keep the gas check plastered to the boolit base.

However, it could certainly be taking a bite out of the heel of an unchecked boolit as the port offers "the easy way out" to the gas column at the instant the heel of the boolit passes. Also, as the port diameter is considerably smaller than the bore, and unobstructed, I'd think the gas velocity through the port would be much higher over a very short term than that of the gas column doing the work behind the boolit.

It seems like some boolit recovery is in order to have a look at the bases.

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Last edited by BD1 on Tue Jun 16, 2009 11:38 am, edited 1 time in total.
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Re: Gas port

Postby Siringo » Tue Jun 16, 2009 7:31 am

I am doing some shooting today on my way out of town -- I will see if I can recover any. I am using a 335 gr. WN from Cast Performance.
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Re: Gas port

Postby wildcatter » Wed Jun 17, 2009 6:37 am

I told ya, youz guys would be all over this. And I thought it to be a worthy question to put to bed. I actually spend a considerable amount of time recovering bullets. Suggestion, I have used two or three lined-up, five gallon buckets, with dirt in them, to recover bullets, with pretty good results. This is way better than going to a public range and trying to figure out which bullet is yours..
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Re: Gas port

Postby Siringo » Wed Jun 17, 2009 11:09 pm

I was shooting some 335 grain WNFP gas checked bullets yesterday. I had recovered a few of them and none had any gas checks attached -- although the bases look OK. However, I notice something that I had seen before when shooting cast bullets thru the 450 -- the bullets actually slugged into the bore. There are rifling traces along nearly the full length of some of the bullets. These bullets are very hard alloy and I am suprised that the nose section slugged into the bore. I was using 30 grs. of LG and the velocity was in the 1790's. Pic's to follow.
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Re: Gas port

Postby BD1 » Thu Jun 18, 2009 8:05 am

When you say the boolits are "hard", do you know the BHN? Large base diameter, fairly quick powder = sudden pressure peak with a large hydrostatic "advantage" against the boolit. It could be that we need to heat treat?
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Re: Gas port

Postby Siringo » Thu Jun 18, 2009 5:07 pm

These bullets are from Cast Performance. Website says they are heat treated and BNH 18-21.
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