Preparing to build a .300blk pistol, thoughts?

[Balog]

A while back I bought two stripped Spike's lowers, intending to build one up and hold the other in reserve for future projects/interests. Well, the more research I do the more I want to build up a 10-12" barreled pistol in .300blk with the Sig arm brace on it. WA law prohibits both SBRs and carrying a loaded rifle in the car, so this is a handy way to comply with both of those laws while still having a short and powerful weapon with good capacity available while traveling. No intention of getting a suppressor for the moment, so subs are not a part of my plan. Here are my thoughts, let me know if I'm off base here.

Lower: virgin stripped Spike's.
Receiver extension/buffer tube: generic mil spec A2 for length and function. Plan to use standard rifle spring and buffer as well unless there's a reason not to?
Sling loop/retainer plate: my understanding is that this will use up thread space and prevent the A2 buffer from correctly engaging the takedown spring detent thingy. Spike's already have this hole tapped, so I plan on clipping a bit of that spring off and using a small cap screw to retain it.
PSA LPK I have laying around
Magpul MOE grip
Sig brace

Upper: Blem Aero's are cheap, and since they're local I may be able to pick one up directly and save on shipping.
Barrel: looking at the Rainier stainless barrels at AIM Surplus. I wish it were carbon and melonited instead of stainless, but it's not a deal breaker. Most of the cheaper ones I see online are 16".
BCG: probably buy the AIM budget 9310 one unless a semi instead of FA profile is needed?
Not sure what to do for a handguard. I want to be able to put an AFG on there eventually, but I'm not terribly worried about free float etc and I don't really want a full quad rail. Will any of the Magpul hand guards work on that short of a barrel/gas system?
Gas block: whatever decent quality one I can find on sale. Do I need a special model to work with the pistol length gas and .300blk caliber?

[tallburnedmidget]

A while back I bought two stripped Spike's lowers, intending to build one up and hold the other in reserve for future projects/interests. Well, the more research I do the more I want to build up a 10-12" barreled pistol in .300blk with the Sig arm brace on it. WA law prohibits both SBRs and carrying a loaded rifle in the car, so this is a handy way to comply with both of those laws while still having a short and powerful weapon with good capacity available while traveling. No intention of getting a suppressor for the moment, so subs are not a part of my plan. Here are my thoughts, let me know if I'm off base here.

Lower: virgin stripped Spike's.
Receiver extension/buffer tube: generic mil spec A2 for length and function. Plan to use standard rifle spring and buffer as well unless there's a reason not to?
Sling loop/retainer plate: my understanding is that this will use up thread space and prevent the A2 buffer from correctly engaging the takedown spring detent thingy. Spike's already have this hole tapped, so I plan on clipping a bit of that spring off and using a small cap screw to retain it.
PSA LPK I have laying around
Magpul MOE grip
Sig brace

Upper: Blem Aero's are cheap, and since they're local I may be able to pick one up directly and save on shipping.
Barrel: looking at the Rainier stainless barrels at AIM Surplus. I wish it were carbon and melonited instead of stainless, but it's not a deal breaker. Most of the cheaper ones I see online are 16".
BCG: probably buy the AIM budget 9310 one unless a semi instead of FA profile is needed?
Not sure what to do for a handguard. I want to be able to put an AFG on there eventually, but I'm not terribly worried about free float etc and I don't really want a full quad rail. Will any of the Magpul hand guards work on that short of a barrel/gas system?
Gas block: whatever decent quality one I can find on sale. Do I need a special model to work with the pistol length gas and .300blk caliber?

Hi there and welcome!

For the most part I don't see any issue with your proposed setup. Get a low profile gas block that reviews well for fitting under hand guards. some out there have problems fitting under handguards so reed reviews on midwayusa before buying. I went with the most plain jane low profile gas block that had zero complaints about fitting under a hand guard.

As for handgurds, I am in love with Midwest Industries (MI) Gen2 SS handguards. The Carbine length is 7 inches. These handguards are 1 piece, very very light, very compact, free float, and rock solid. They use a standard barrel nut, have a single built in top rail with ability to custom bolt rails to your hearts content, and are so simple yet elegant in design that it is pure genius. You can install/remove the hand guard with an allen wrench while never needing to touch the barrel nut, gas tube/key, or gas block. Wonderful for modification purposes.

Additionally, I just put in my eForm1 to SBR my 300BLK pistol because it was cheaper for me to pay the $200 tax stamp (I already have a trust from my suppressor stamp) and use an optic and rings/mount I have lying around than to buy the longer eye relief optic I would want/need for holding the pistol away from my body while shooting. SBR'ing solves this issue for me for less money hahaha.


I hope this info helps and good luck with your build

[Balog]

That's very helpful, thank you. I've seen the YHM 9383 recommended, but it comes in a couple different versions. I've also thought of just getting a normal clamp on and cutting off the top after roll pinning it, as I've heard they tend to be the most stout.

Great thoughts on the rail, I really like Midwest Industries so it would be nice to give them some money.

[GSO]

Welcome to the forum.

I would have one concern with your proposed set up, although I may just be paranoid. If you use a standard A2 rifle buffer tube, some overzealous law enforcement types might claim you had "constructive intent" if you ever had any LEO interaction with this pistol. It could be argued that you could easily remove the SigTac brace and install a standard A2 buttstock to create an illegal SBR. It might be wise to use a regular "pistol" buffer tube. It won't be quite as long as the A2, but should be close.

I used the Phase 5 pistol buffer tube complete assembly from Brownells and it works great. I did substitute a GG&G ambi sling loop instead of the standard carbine end plate, so that would also work for your application without having to mess with the buffer retainer. The shorter length of threads would affect the buffer retainer rather than the takedown pin detent and spring. Also, I'm not quite sure any of the sling loop retainer plates would work with a rifle buffer....I'm almost certain they will only work with pistol and M4-type buffer tubes. So the point may be moot.

http://www.brownells.com/rifle-parts/st ... 54427.aspx

I'm still waiting for my tax stamp, but in the meantime, I have put a SigTac brace on mine and it works pretty well. I can post or PM a picture later (when I'm home) if you want. And at risk of appearing to pimp my wares, I am a part time FFL and have 3 extra SigTac braces for sale if you still need one. I could probably do it for $115 shipped if you are not in Texas since I wouldn't have to recover sales tax.

I've never used them, but heard good things about Midwest Industries hand guards, so I don't think you can go wrong there.

Finally, you would want a lo pro gas block since you'll have a pistol length gas system and carbine length hand guards. Easiest solution and sturdiest is a lo pro already set up for pinning to the barrel, like the Geissele below. A regular lo pro should work fine, and isn't likely to work lose if covered by hand guards, but the extra bit of security that the pin provides wouldn't hurt if you were motivated and had the tools to drill the barrel to accept the pin after mounting the gas block.

http://www.brownells.com/rifle-parts/ga ... 56211.aspx

Hope this helps.

[300Blk]

Constructive intent - the theory that if you own a can of gasoline somewhere in your house, and also own matches somewhere else in your house, then you intend to perform arson.

You have a very good reason for using the receiver extension that you already own - you already own it, and buying a pistol extension would cost more money. Take some racquet tape or bike handlebar tape and wrap it up good. Sure, someone can argue that you can remove the tape and put on a stock, but you could also change the extension.

There are "constructive intent" theory people who will tell you to not even have a short barrel upper in your house unless you own an SBR or full auto lower. It is not illegal to have one, and we should not give up even more rights by avoiding doing legal things.

That being said, I think there is an issue with using an A2 - it is longer than needed to house the buffer - and you just cited that you liked the length - presumably to use it as a stock - so one can argue that it was chosen to act as a stock even without an explicit plastic stock. So for that reason alone, it is better to have a pistol tube which is clearly not and has no unneeded extra length.

[GSO]

Constructive intent - the theory that if you own a can of gasoline somewhere in your house, and also own matches somewhere else in your house, then you intend to perform arson.
...

That's why I hesitated to bring it up. Too vague and mostly just paranoia since you have to actually have an LEO "interaction" for it to even come into question.

I think the big practical argument for using a dedicated pistol lower is that the sling loop plate will work better with the pistol tube.

[Balog]

Partly it's length, partly it's just cheaper to get an A2 tube than a pistol. What is the actual length difference? Are they necessarily even marked differently?

[GSO]

Partly it's length, partly it's just cheaper to get an A2 tube than a pistol. What is the actual length difference? Are they necessarily even marked differently?

About 2" difference in length, although there is some variation in pistol buffer tubes. The Phase 5 that I linked to is 7-3/8", and a standard A1/A2 is about 9.5" or 9-5/8" depending on the source. I don't have one in front of me to measure.

The Phase 5 tube is clearly marked "PISTOL" so there is no confusion. There is also no threaded hole in the end for bolting on an A1/A2 stock.

The other big difference is that the pistol buffer tube has a slot milled through the threads along the buffer axis just like a carbine buffer tube, uses and end plate, and has a castle nut to hold it all in place. This is needed for the end plate that goes with either the pistol or carbine buffer. The A1/A2 buffer tube does not have this slot nor does it use an end plate or castle nut. You stated that you wanted to use a sling loop end plate, so this would be the big reason for using the pistol buffer tube.

I'm a big guy...6.2", 37 inch sleeve length...buying shirts is a pain. I put the SigTac brace about half way on the tube before it became difficult to move, even with baby powder. The length of pull is reasonable without an extra 2" in the buffer tube.

[AR-300]

I went ahead and ordered a Phase 5 pistol kit
less hassle and no ambiguity

[farm use]

Phase 5 is top notch! I have one now on a 8" pistol. I have used them on other builds for others, they are very pleased with them also.