Can someone give me the quick rundown (or a link) on the advantages and disadvantages of each?

I'm looking to build two match grade uppers (one in 5.56 and one in 6.8) in the very near future.  I haven't fully decided on 18" or 20" for either yet.  Although I have all of the SPR Mod 0 components sans barrel.

-Red-

Quoted: Can someone give me the quick rundown (or a link) on the advantages and disadvantages of each? I'm looking to build two match grade uppers (one in 5.56 and one in 6.8) in the very near future.  I haven't fully decided on 18" or 20" for either yet.  Although I have all of the SPR Mod 0 components sans barrel. -Red-

For a 18-20", the rifle length gas system is probably better that the mid-length. The mid-length shines in the 16" carbine format. In fact for 6.8SPC, you really need the mid-length in 16".  Wes Grant @ MSTN is probably the best source of info. All of the 16-16.5" uppers I have had built are mid-length both in 5.56MM and 6.8SPC. I am having another MSTN Mod 1 5.56MM mid-length 16" "Recce" built by Wes as we speak.  

link thingy

Quoted: link thingy

I apparently missed this thread. Are you saying that for anything less that 20", the mid-length is the way to go? Similar to the MRP 18" mid-length?

WOA recently built me a 17.5" rifle length upper that runs like a champ....  YMMV

I wish I could get a 18" MRP barrel with the rifle-length gas system.

The 17" barrels with the rifle-length gas system look real good.

-z

Quoted: Quoted: link thingy I apparently missed this thread. Are you saying that for anything less that 20", the mid-length is the way to go? Similar to the MRP 18" mid-length?

No, not really... saying that the original set up (rifle) is the way the gun was designed to work, the mid-length with a 16" barrel is also a very good setup. Builders are forced to use what is out there, or make and offer something non-standard, so there are some trade offs... Moving the gas port around is expensive, when just guessing and drilling -- I have went through more than one barrel with multiple gas ports drilled, welded up, drilled, welded up, on and on and on...

For the most part, a computer model can tell you the gas pressure at different points along the bore, from there it is a balance of tapping the gas, proper port size, pressure time (pt) curve, dwell time, mass of moving parts, buffer system and lock time... My bet is the best standard is the peak internal pressure in the bolt, there are some high speed video shots that show some detail of the gun working... but the pressure is probably the best measurement -- but difficult to measure.

For the most part, the systems on the market are generally "works for me" development and tend to work fine, opening the gas port is a safety step that ensures the gun will run... and for a semi auto is not so bad -- But too high of a bolt velocity can play hell on a running full auto and introduce a host of problems (most of which are well known by now).

Anyway, I have probably rambled on too long...

Quoted: Quoted: Quoted: link thingy I apparently missed this thread. Are you saying that for anything less that 20", the mid-length is the way to go? Similar to the MRP 18" mid-length? No, not really... saying that the original set up (rifle) is the way the gun was designed to work, the mid-length with a 16" barrel is also a very good setup. Builders are forced to use what is out there, or make and offer something non-standard, so there are some trade offs... Moving the gas port around is expensive, when just guessing and drilling -- I have went through more than one barrel with multiple gas ports drilled, welded up, drilled, welded up, on and on and on... For the most part, a computer model can tell you the gas pressure at different points along the bore, from there it is a balance of tapping the gas, proper port size, pressure time (pt) curve, dwell time, mass of moving parts, buffer system and lock time... My bet is the best standard is the peak internal pressure in the bolt, there are some high speed video shots that show some detail of the gun working... but the pressure is probably the best measurement -- but difficult to measure. For the most part, the systems on the market are generally "works for me" development and tend to work fine, opening the gas port is a safety step that ensures the gun will run... and for a semi auto is not so bad -- But too high of a bolt velocity can play hell on a running full auto and introduce a host of problems (most of which are well known by now). Anyway, I have probably rambled on too long...

Yes, you may ramble at times....