So a friend of mine needs to sell an AR that he built. Its a DD upper reciever, DD Quad rail, DD 16 inch barrel(carbine gas) with a Adam Arms Piston Conversion. With an M&P 15 Lower receiver, buffer tube and Magpul ACS stock.

I need another SBR and I was thinking of getting it from him then selling the lower and barrel and buying a a 10 inch .300 BLK barrel and throwing it together. Will that work okay, would be primarily suppressed. Same question for a 10 inch 5.56 barrel>

I know nothing about these Adam Arms conversion things. Doing some googling on it now.

I own several different piston systems (Adams Arms, LMT, and PWS) and they all run great. The conversion system should work with the 300BLK as it did with the 5.56 setup as long as the gas block journal is the same size. I am going to assume it's a carbine length piston system. The Adams Arms gas block is adjustable so you will have no problems running suppressed. If you don't mind me asking, what brand of barrel are you looking at putting on it?

ATW!
-Fox

ETA: Adams Arms says the lightened bolt carrier is essential for optimal performance of the 300BLK with their kit.

Yes it is carbine length. I'm thinking a 10.5 inch Odin Works 300 BLK barrel.

I'm assuming the AA Conversion fits .750  gas block journal but haven't checked yet.

Is the current DD barrel govt profile?

ATW!
-Fox

No its not gov profile.

I'm not sure what DD calls it but its their CHF Medium weight barrel I believe. I don's have any pics of it handy.

I did my M&P a couple years back. I love it.
Things to keep in mind:
If it's gonna fit under a rail be sure to get the XLP gas block. It is adjustable and will allow you to tune the gas system for suppressed vs unsuppressed fire (it isn't a simple 4 position gas block.... It's got like 8 or so positions, so you're going to have to find the settings that are going to work for your gun). That adjustment though ought to allow the piston to cycle on whatever length barrel you have.

Took some crappy pictures of it:

Looking at that picture my money is on the gas journal being .750".

ATW!
-Fox

No resale value ...

In reference to what specifically?

W/o a round count it's hard to give an approximate value - but lets for instance say it has <1000 rounds, I wouldn't pay more than $750.

I would not put a AA system on a 300BLK, they don't run well. You can search issues other people have had.

Superlative Arms kit. They have almost limitless fine gas adjustments. They are by far the lightest. They are the ONLY one that off gases excess gas through a pattneted blow off hole in the piston. There was a measured 10db drop in suppressor noise using the SA kit.

https://www.youtube.com/watch?v=Cy5XLr3igaU

Jusdge for yourself from this very comprehensive test. Michael Bell has earlier videos comparing the weight to AA kits in two earlier videos. Those are labeled for Syrac Kits which are MADE by SA for Syrac. But dont get the Syrac kit. SA does not send them their patented off gassing version which is grpund breaking in its performance. Watch the video and decide for yourself. I did the math. You gain 20z and pay an extra $175 over a comparable adjustable DI system that has NO off gassing capabilities.

The price point is the same or LESS than an AA kit.

Micheal Bell has a thread on his SA videos (with links) here in this section of AR15.com
http://www.ar15.com/forums/t_3_126/696041_Superlative_Arms_Piston_Kit_video_review.html

If you stick with an Adam arms piston I would call them and ask what their recommendations are for the gas port size on a 300 blackout barrel. The variation in blackout carts can be problematic for piston blackout combinations. i  had a ballistic advantage barrel port that I had drilled out to the recommendations for the piston company. It works now with subs and supers. You'll love blackout

Need to ream out the gas port hole to make it work. Since 300BLK round is sub-sonic.
Hope this help.