Looking at building a SPR type rifle. Been asking all kinds of questions. I am looking for a 18" barrel. I have read that a 16" with 1:8 twist with mid length gas was used on the org recce rifle because it was the best all around twist rate for all rounds in use(55gr to over 70gr). I am not looking at starting a twist rate debate here and I know the Mk12 SPR had a 1:7 twist.
THE QUESTIONS ARE––1.what would change if anything by adding to the barrel and putting a rifle length gas system on it? So basicly what I want is a 18" 1:8 twist barrel with a rifle length gas system. 2.What would the effect be if I used a Mid length gas system instead of rifle? 3.What effect if any would this have on the performance? So far I have 3 companys that make these.
Definitely not the expert on these things, but from my own research the difference between mid-length and rifle-length would be a reduction in dwell time between the gas port and muzzle. The reduction will cause less time and force to cycle the action as the gas gets weaker the farther it expands. You might need an increased size gas port to allow as much gas through the system before the bullet leaves the muzzle.
Some places say the difference between 1:8 and 1:7 twist is 1:8 can handle up to 75gr bullets and 1:7 can handle up to 80gr - but 80gr is too large for standard components and requires a properly throated chamber.
An 18" 1:8 twist rifle-length gas will work just fine. Many people here prefer the longest gas sytem possible as it still allows the action to cycle reliably and reduces felt recoil. I would recommend getting an easier to acquire, quality, 20" 1:8 barrel with rifle-length gas. Then send it off to the barrel-smith of your choice to have the barrel chopped to the desired length. Many here prefer ADCO for such surgeries as they are quite experienced and know when other modifications like an enlarged gas port are needed.
And if you're doing an SPR build, get a heavy barrel. When you have it chopped down, get it fluted as well to reduce the weight a little but keep the strength.
The more scientific aspects of your questions I'm sure will be answered by many others below me. In the end, enjoy the build and shoot the shit out of it when it's finished. =)