Really interested in doing a pistol build next.  Considering .300 blackout after all these great threads.  However, need some help from the hive on a few things.

-can I use a 5.56 lower for the build?
-subsequently, can I use my current 5.56 mags?
-is an 8.5 barrel that much different than a 7 inch.  
-I think the answer is no, but would my Gemtech G5 be compatible with the .300 round?

I'm going back and forth with making this a 5.56 build, but I already have a 10.5 SBR, But based off many threads, I'm guessing  a 7 or 8 inch barrel with 5.56 isn't a good option.

Thanks team.  With the 4th of July sales going on, I can't pass up the deals on uppers right now so looking forward to starting this build!

Happy Independence Day!!

I cannot comment on .300 Blackout, as I do not have one or have ever shot one, but I do have a 7.5" 5.56mm and it shoots and works outstandingly.  Most of them do.  Its one of the more popular barrel lengths for a pistol.  Don't let the length be a problem as it works just fine.  Same thing I tell the womenz, LOL.

only thing different is the barrel.  I have a 8.5

The only difference between 5.56 and .300 is the barrel. The 8.5 inch will allow the threads to be beyond a 7 inch hand guard if you want to attach a can that doesn't fit into the HG. 

I am in the midst of gathering parts for a similar build.  My 8.3" barrel will leave just the threads of the barrel sticking out of a 7" handguard.

I wanted to do a 7 ish inch barrel but found a great deal on the 8.3" and think it will work better.

Thats the great thing about .300 BLK it uses ALL 5.56 components except for barrel.  I am not sure about the suppressor but you may need to get .30 end caps for it if its a 5.56 suppressor.

Please correct me if I am wrong, but wouldn't you need a .30 cal suppressor for 300 BLK? I'd imagine a 5.56 suppressor would be for .22 cal. and smaller.

300BLK requires a .30 cal can. Also, the threading on the 300 BLK barrel is 5/8"x24 vice 1/2"x28 as found on 5.56/.223 barrels.

The optimum designed length barrel for 300BO was/is 9".  However, 8.5" and 9.5" are probably almost unnoticeable as far as any performance degradation.  Now, I'm mainly talking about suppressed shooting, and that's where the application of 300BO really shines IMO.  If we're talking about unsuppressed with 300BO, I think you'd be better served with a 16" barrel solely for shooting supers.  Stay close to the 9" barrel for 300BO...especially suppressed.

Magazines fit both 300BO and 5.56, but I'd recommend some highly recognizable markings for the 300 stuff to avoid disaster.  I use polymer mags for 5.56/.223 and metal mags for 300BO.

On barrel length for 5.56 pistols, some very wise and experienced AR15 experts tend to draw the line at your 10.5" 5.56 SBR.  Most ballistic charts tend to support that.  If performance is more critical to you, I'd pass on a super short barreled 5.56.  For just a fun plinker with free fireballs in low light or dark, it probably won't matter.

Some 5.56 mags have issues running .300blk with the 200-220 gr. bullets but my PMAGS, E-Landers and Brownells GI mags run 125-150 gr. fine. Lancers seem to run everything.

-can I use a 5.56 lower for the build? - Yes, I run an AM-15 Anderson lower and Aero lower and they both run fine
-subsequently, can I use my current 5.56 mags? - Most will run fine; I run a majority of PMAGs and have never had an issue. so you should be fine
-is an 8.5 barrel that much different than a 7 inch. - I would recommend 8.5 but I would focus more on twist.  If you're going to run heavy subs through it I would recommend a 1:7 vs 1:8.
-I think the answer is no, but would my Gemtech G5 be compatible with the .300 round? - Need a 30 cal can

Best of luck and Happy Independence Day

I looked up ballistics with .300 aac supersonic ammo and decided 10.5" was the shortest I wanted to go for my pistol.
I have also added an extended buffer tube to get it over 26" so I can have a vertical fore grip.

Roughly 2100 fps with a 110gr bullet. This is with factory ammo. Hand loading might be able to bump that up a bit.

Only thing you need to change is barrel and muzzle device due to different thread pattern/caliber.

I ended up with both an 8.5" and a 10.5" upper. Why not? Got each of them for under $300. The PSA came with the BCG, as it was a used unit on GB. The only thing I do not know, is if the PSA 8.5" upper is a 1:7 twist or a 1:8 twist. Everyone seems to say go to the 1:7 with heavier bullets, which is what a lot of the subsonic loads are loaded with. I know the 10.5" is a 1:8, as it was purchased new from CDNN.
 
Hope you are feeling comfortable with your decisions.....finish the build?

BTW, I went with a few "tactile" differences in my build, like a finger grooved MAKO OD green pistol grip and OD foam buffer tube cover. I have no OD green on any other rifle or pistol for that matter.

And I decided to use OD green Hex Mags for my .300 as it has a distinctive texture I will know even in the dark. I tend to keep pre-loaded magazines.

Quick question for you on this.  I am also running USGI and PMAGs on my 300 blackout build with no issues, however I noticed that the magazine won't seat in the receiver unless the bolt is back.  I don't have this issue with my .223 AR.  Is this due to the size of the bullet, or is my AR just finicky?

I'm almost certain the 8.5" PSA has a 1/7 twist.

Every one I've seen is at least.

Sorry for the delay I was out of town.  What lower are you running? Is it only PMags or are the USGI mags causing problems too? Does this issue happen with subs only, supers only or both?  Also what ammo are you running?

I've used 3 lowers with 2 300blk uppers and a variety of magazines no issues seating. Try downloading mags by 2. Also they can move around alot so tap the spine of the magazine on your hand and keep all the rounds seated all the way to the rear.

GI mags work best from what I hear but everything I've tried so far has worked.

Per loading doctrine you DON'T shove a mag in unless the bolt is back, regardless. It requires you to compress the bullet stack against the heavily loaded mag spring, it might fit with friction spreading the mag body but can fall out if the mag catch isn't fully engaged. In other words, bad practice. Always retract the bolt.

Some say there is a tactical application where you reload to a full mag while still needing a round in the chamber, fine, your SWAT/SERT Team policy as enforced by your instructor will apply. Meaning that you likely will never need to do that shooting dirt on a public range. Swip swapping partially loaded mags is something for tactical experts which there are actually very few.

As for the .300 BO using "all 5.56" parts Magpul disagrees - and just offered a .300 specific mag. Why? Because the width of the bullet stack just forward of the cannelure is larger with the bigger .300 bullets compared to .223, and that means they rub the front rib and cause friction, including jams and stoppages. Since there is a real danger of mixing rounds in guns, having a .300 specific mag marked for that use is a necessary safety measure. It will also help feeding as some have noted, 5.56 mags are not optimal and will cause issues. We knew this all along but the popular myth about "same as 5.56" is so spread around it's reached mythological proportions.

So, Magpul has the final say, and those are the only .300 BO mags on the market now. Everything else is taking a chance.

My method for avoiding confusion between 5.56 and 300BO is to use steel C-Products mags for all my 300BO and retain my polymer...Magpul, HK, Lancer, etc...mags for 5.56/.223.  It achieves another benefit in that the steel mag has a little bit more clearance for longer COAL for the longer, heavier subs.  Also these steel mags don't have that rib in front, so no interference with the big, long subs.  I've had some fun shooting some cottontails on the run at my house doing rapid fire with no malfunctions using subs and suppressor.  These steel mags are working just fine with 300BO for me...and no confusion about what bullets are in what mag.

Not sure if serious...


But you can you buy all those fancy 300blk-specific mags you want. I've got a 16" and 10.5" and have used Troy, Magpul (gen 2 & 3, 10/20/30 rounders), Lancer (20 & 30 rounders), Hexmags, and aluminum mags, all with no issues. The only issues I've ever had have been ammo related (Nosler 125's blowing primers like it was going out of style). I only shoot supers, not sure if there are specific issues with subs with relation to certain mags. Either I'm one of the luckiest SOB's around or these mag "issues" aren't as wide spread as some would have you believe.

You haven't seen the D&H .300blk mags? They were out before Magpul got into the game.

I have a few D&H .300s but so far I've not found any commercial subsonic ammo that won't run in my Lancers, my E-landers, or my Brownells GI mags.

D&H does use a red follower to help identify it as .300blk but if I buy more 'dedicated' .300blk mags they'll be more Lancer transparent FDEs. I only use that color for .300blk and since they're transparent I can see EVERY round if I want to confirm it's all the same.

I bought some magbands to easily identify my 300 BO mags from my 5.56 mags.  I like this non-permanent way of marking my magazines.

https://farm5.staticflickr.com/4256/35652129201_c27f84dc50_b.jpg

metal mags for 5.56 can rub the bullet on blackout, so some guys prefer poly mags and stay away from steel mags.

you are free to run everything thats mil-spec/standard, only need to swap the barrel.

blackout can go down to 4.5 or 6 inches in barrel length running heavys effectively, but at that point you may need to do more than just swap a barrel.

personally the 10.5 with polygon twist is pref, runs just about anything jacketed, on any length tube*...