Barrel/gas system length

[stroo]

It looks like the 16" barrel has a carbine length gas system and the 20" has a mid-length gas system. I am just looking at pics, is this and true? And is there any difference in recoil between the two?

[2zero6]

No,there are actually 2 gas blocks. the forward one is just for looks, there is a carbine length gas tube and low profile gas block under the hand guard. I wish they would have put the railed one they use on the 16" on the front of the 20" so I can put my folding BUIS on it.

[thebrassnuckles]

wanna trade?

[Hoot]

Did they (Bushmaster) ever explain the logic behind that decision during the development, because it so does not make sense? Any other AR with a 20 inch barrel has a rifle length gas system. Would it be a function of the fact that the .450 is such a large bore volume compared to a .223 system and they simply run out of pressure before the bullet would reach the port location for a rifle length system? A quick and dirty calculation looks like 4 times the volume simply comparing two barrels or equal length with one being .223 and the other .450 diameter. Is that just the price for the velocity because a slower burning powder would peak when the bullet was further down the barrel? I assume Bushmaster tried a lot of different combination's of powders and barrel lengths before deciding they had reached optimization, or was the 20" an afterthought?
Hoot

[2zero6]

wanna trade?

do you have a 16" or 20" and how many rounds fired through it?

[wildcatter]

Did they (Bushmaster) ever explain the logic behind that decision during the development, because it so does not make sense? Any other AR with a 20 inch barrel has a rifle length gas system. Would it be a function of the fact that the .450 is such a large bore volume compared to a .223 system and they simply run out of pressure before the bullet would reach the port location for a rifle length system? A quick and dirty calculation looks like 4 times the volume simply comparing two barrels or equal length with one being .223 and the other .450 diameter. Is that just the price for the velocity because a slower burning powder would peak when the bullet was further down the barrel? I assume Bushmaster tried a lot of different combination's of powders and barrel lengths before deciding they had reached optimization, or was the 20" an afterthought?
Hoot

Sometimes what doesn't make sense, does, if you were on the inside and if your not, a question is sometimes smarter than a statement.

So here's the skinny, short and sweet.

Bushmaster did not explane how they arrived at the decision, you are wondering about, but I'll give you some insight. First realize, that what you think is crazy, is also a system that works and works darn well, wouldn't you say?

The Carbine length Gas block is located where it is, because the barrel port pressures the 450 produces, are far lower than the 223 produces, when both the 223 and 450 are measured at the rifle length port position.

The powders for both are worlds apart, in how much they expand, rate of expansion, and other factors, compared to each other, hence for any given port dia, the slower powders produce more port pressure. Example; According to std burn rates, lil-gun is an extremely fast rifle powder, normally better suited to pistols, where as, the normal powders for the 223 are medium-slow rifle powders (yeah i know this is over simplification, but I don't want to lose some readers).

The cure could be to open up the port for the 450, to get more gas going down the gas tube, but then the port is way to large for reliable, long term operation or... You could use more powder in the 450, to increase the port pressures, but for you that might be a bad, dangerous choice and must be left to the more adept. Or. You could use very slow powders, but then nobody would be happy with the slow bullet speeds (hey, this ain't no socom) and the tremendous sooting and it's accompanying fouling. You could relocate the port closer to the chamber, where the pressures are higher, problem solved.

So here's the deal, there are folks that complain about the need for a Corvette, but most around here run around with their hair on fire and just understand why such things are necessary. So, the real question is, do you want to drive a Toyota or a Corvette, both run down the road equally as well.. or do they? ..t

[2zero6]

I am inclined to agree with you Wildcatter, either way current port position and size work. I am not a powder or projectile engineer nor claim to be, so changing port position and/or size is not an option. I am however willing to give up the 100 or so feet per second by going to the 16" for the added maneuverability in the bush or home and as I am not looking for a bench rest gun with this cartridge. Sooner or later I will get into reloading for the $ savings aspect in training/practice but for now I am really happy with the factory loadings (especially when the new Remy AccuTips come out). So Stroo I guess to each his own and barrel length is a matter of what is your intended purpose is with the rifle/carbine. Thanks again for your input Wildcatter I always value your insight.

[stroo]

Thanks for the input guys! I just changed my order from the 16" to the 20" for the little extra velocity/range. My primary purpose will be deer hunting from a stand and I sometimes hunt the edges of fields so that extra 100fps will hopefully pay off against the maneuverability of the shorter stick.
Does anyone know if the 'fake' gas block on the 20" is functionable? Is the barrel finished under it? Thinking about removing it and using it on another build...

Thanks again,
Rick

[Hoot]

Though I'm waiting on my upper (4 weeks today) I have read that the front block is non-functional.
Hoot

[thebrassnuckles]

wanna trade?

do you have a 16" or 20" and how many rounds fired through it?

i have a 16"

it has fired a grand total of 5 rounds.

i was talking about just the gas block.. not the whole upper. i like my 16"