BCM 14.5" midlength gas port size?

AR/M4 AR Technical Discussion
1

I’m curious if anybody had the gas port size for BCM’s 14.5" middies? Mine runs like an absolute champ, for the record.

I see so many of this config on the market now. But it seems that BCM and Noveske were the first to really use the configuration on a mass produced level. I am uncertain if noveske conducted cyclic rate tests like bcm did. Regardless, I am interested in the bcm gas port spec.

I’m guessing it will be in the high .06’s or low .07’s.

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2

I should have a new stripped BCM 14.5" middy, lightweight profile barrel in a few days. The inspection department where I work has a pin gauge set, so I can get an exact measurement for you. I will post back with the measurement later this week. I don’t believe there should be much difference in port size between their .625" gas block or .750" gas block 14.5" length middy barrels.

3

Biggy, thank you sir.

I think I remember Ned Christiansen commenting on the sizes varying with large changes in gas port length, i.e. a large change in the gas journal shoulder. I would think that, if there is a dif, it would be minimal.

I look forward to your report.

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4

There is a difference, not much but there will be one. The smaller (.625) barrel will have a slightly smaller gas port.

5

I took these 4, brand new, stripped, unfired, mid-length gas barrels to work tonight and checked each ones gas port size with the inspection departments pin gauge set. These are exact measurements down to .001" of an inch.

BCM 14.5" CHF lightweight profile, .625" gas block: .077" gas port (I just got this barrel from Bravo Co. yesterday)
Centurion Arms 16" CHF, .750" gas block : .079" gas port
Noveske 16" CHF, .750" gas block : .080" gas port
Rainier Arms 16" Ultra Match, .750" gas block : .082 gas port

6

You, sir, are a gentleman AND a scholar. Thank you. :smiley:

7

Good data, thanks

And GO BUCKS!

8

Thanks Biggy.

BCM recommends an H buffer while Noveske recommends an H2, if I’m not mistaken. That’s a small difference in gas port size for a considerable difference in buffer weight. Interesting stuff.

Thanks again.

edit - not sure of the differnce with a standard profile BCM mid length, but I’d assume it’s a larger port. That would make the difference even less.

9

.003" difference in diameter equals an 8% increase in cross-sectional area. Fairly significant.