SBR and the midlength gas system

[rob_s]

I may be misunderstanding what you mean when you say "midlength".

I was just saying the "shortest" length i've seen on an AR is 7".

A standard AR (or military M4) carbine is 7"+/- from the front of the upper receiver to the rear of the front sight base. Hence, the Daniel Defense, Larue Tactical, and other makers "7.0" rail for standard AR carbines.

A midlength gas system pushes the front sight base out another 2"+/-. Hence the "9.0" rails from Daniel Defense, Larue Tactical, etc.

Having a 7" barrel on a 9" gas system would be a little difficult. In fact, having a 7" barrel on a 7" gas system would be kind of tough as well. Most 7" barrels push the gas block back to somewhere in the neighborhood of 4-5" from the front of the upper.

[A62Rambler]

Is shortening a middy to 10.5 to 12.5 doable? Yes (It may or may not function reliably)
Is it more trouble than it is worth? That's for you to answer not us. The gas system on the AR requires the bullet to be in the barrel beyond the gas port for some amount. On a carbine it is often reported that for reliable performance you need a 10"+or- barrel to accomplish this. That would lead me to believe that you would need a 12"+or- barrel with the middy. If you think it is worth the money savings to experiment and end up with a maybe reliable maybe not SBR that's for you to decide. If you want a 10.5" barrel and you want to use your middy, go for it. You can let us all know what gas port diameter you ended up with and if you needed an adjustable gas block. When you get it all worked out you'll know if you saved any money. If you do this let me know then I'll be able to actually save money because somebody else did all the experimentation.

[Robb Jensen]

For a reliable SBR 13.6-7" is about the shortest you can go on a middy.

[blkexp98]

I am currently thinking about what barrel length i want for a SBR and was planning on using a ~12.5" barrel with a midlength gas system. How i was looking at it is it would pretty much equate to a 10.5 as far as the location from the end of the barrel.

Some of the benefits would be (if it worked ):
-longer sight radius for a fixed front sight (without being set up as a dissapator)
-Im able to recycle my midlength handguard
-12.5 gives more velocity than the 10.5

I see allot of people go with the 12.5" in order to allow certain suppressors to be used but past that i dont see why the carbine length system is manditory. Then again i have never delt with a short barrel so its just speculation. I would think that some gas port tweaks would be an issue though.


gotm4: What is it about the midlength 12.5" long that makes it any different than a 10.5"? Im not saying your wrong or anything i was just curious since i was making plans to have the barrel cut.

SethB: What size gas port is nessisary to make a midlength work?

Overall i like my midlength gas system on my 16" ar and thats another reaon i wanted to use it for a SBR barrel. The thing is i wouldnt want to go any longer or shorter than 12.5". I think much longer than 12.5 will not give the returns of a SBR. 5.5" less than my current barrel would be nice though considering my suppressor is around that length.