I have an Sig Sauer M400 ENH Patrol rifle (RM400-16B-ECP-FDE) with the standard 16" barrel.  I would like to convert this to a 20" barrel.  Anyone have any thoughts on what would be the best barrel for this rifle?  Also, assuming I replace the gas block and gas tube with the long tube and block for that tube, I am hoping to not have any problems?  I will of course speak with my gunsmith but was wanting anyone who has done this to chime in if possible first.  Reason = effective range with M193.  2700 fps is the velocity that facilitates bullet yaw.  2700 FPS with a 16" barrel is about 100 meters, 150 meters for a 20" barrel.  Thanks in advance.

Best barrel is somewhat subjective, as well as depending on your budget and how much weight you want to add. Provide more info.


It's far better to buy or build separate guns for intended purposes. There's no goldilocks build that does literally everything great. If your entire goal is killing bad people with M193 up to 150 meters, you can do that with a 10.3" barrel. Or maybe you could use ammunition that is actually good, like bonded soft points. All that extra weight of a 20" barrel over a 14.5" or 16" can be used for adding an optic, laser, white light, etc. Far more useful items than another 4" of barrel.

It's not really a big deal . I just turned an 8.5 inch 300 blackout pistol into an 18 inch 5.56 rifle . It took about an hour including mounting the optic . As far as the best barrel goes would depend on your budget  .

20" barrel with rifle length gas is very easy to get right. Lots of good barrel manufacturers, it's more about specifics (profile and that type of thing).

However, I'd advise against dismantling the existing upper. Provided you can get parts, I'd get the new upper built/bought and keep the 16" upper as a spare, or for a future rifle.

I personally love 20's and have been handling much longer, heavier guns my whole life (28" hunting shotguns etc) but I think having both is ideal.

Thanks for the responses guys.  I had a feeling that two separate rifles was the way to go but figured I would get everyone's two cents prior to making a decision.  The other point about JSP loads is also something I was planning, prior to the ammo shortages.  I have plenty of M193, but no JSP (at least in 5.56).

Another option I would consider seriously (if ammo wasn't in such short supply) is to get an AR10 for that 'reach out and touch someone' ability.

Don't forget that there are a lot of other options besides 223/556 for AR15s that do really well at the type of ranges you're talking.