Building your own : what pitfalls to avoid

[Tisto]

I have built several AR 15 & AR 10 platforms from both 80% and stripped factory lowers, with great success so far. There have been the expected screw-ups, especially with the first 80% attempt. Since then, things have gone smoothly, and quality keeps getting better.

For this next project, however, I'm looking for some advice. I have a dedicated 9mm 80% lower to build out. Although my jig would not accommodate the 9mm lower, I have ordered one that will. Machining the lower is not my problem. I'm looking beyond that. For those of you that have built dedicated 9mm AR's, please help me select parts that will work properly. The upper is already built, and It has a 11" barrel.

As far as parts go, I have a standard AR 15 lower parts kit to use, which should be 99% compatible as near as I can figure. The buffer tube, spring, and buffer have me questioning what I should order. I would like to use a standard carbine tube if possible. I believe that I will need a very heavy buffer, as the upper is of blowback design. I also believe that the buffer needs to be heavier depending on the ammo used. Heavy bullets in +P rounds would require more weight, correct ?

Next, I believe that there is no last round bolt hold open feature. At least, I don't see any. Is there anything special I should do to the bolt catch to make it easier to apply manually ?

Your thoughts and suggestions are appreciated. I don't really want to wind up with a giant box of spare parts via the trial and error method.

 

[BDinPutnam]

Paging @PutnamCharlie - I belive he built one from an 80%.

I built an AR9 but not from an 80% (I used a QC10 dedicated Glock lower). Buffer weight is a factor with the 9mm. I used heavier springs too, iirc. Ran very nicely but I sent it to live in Michigan where it can run free with legit AR furniture and 33 round mags. Very, very fun gun either way.

 

[squib]

The buffer should be heavier to help slow the blowback bolt timing , the spring and tube can be carbine length.

What magazines are the dedicated lower designed for? Does the lower have an integral feed ramp?

 

[PutnamCharlie]

Yea I built one from an 80% and a foxtrot mikes upper. The buffer I believe was a 308 buffer but If not it was heavy. I ended up buying a stripped receiver because of the 80% crap going on. It shoots really nice and it's a great range toy.

 

[squib]

Yea I built one from an 80% and a foxtrot mikes upper. The buffer I believe was a 308 buffer but If not it was heavy. I ended up buying a stripped receiver because of the 80% crap going on. It shoots really nice and it's a great range toy.

How did you like the side charging upper? The dedicated lowers look really slick with the hold open linkage.

 

[PutnamCharlie]

I like the side charge. The only big issue with them is the blowback. You don't really notice while your shooting ( I have only shot it at an outdoor range) but when you get home you can feel it in your throat. I couldn't find their lower so mine is not lrbho.

 

[Tisto]

The buffer should be heavier to help slow the blowback bolt timing , the spring and tube can be carbine length.

What magazines are the dedicated lower designed for? Does the lower have an integral feed ramp?

This particular setup is designed for Glock mags, and there is no feed ramp. The mag positions the cartridge directly in front of the chamber, perhaps 1/8" away. I suppose there could be a feeding issue with wadcutters or other flat nosed bullets, but it looks like anything even remotely round or pointed should chamber without an issue.

 

[Tisto]

From what I can find, I'm going to need a buffer in the 5.4 to 5.8 ounce range. I only found one reference to a different spring unless the super short pistol length tube was used. I can't find any specs on the power ( compression force ) of any springs, so I don't know how to compare them.

 

[repete]

From what I can find, I'm going to need a buffer in the 5.4 to 5.8 ounce range. I only found one reference to a different spring unless the super short pistol length tube was used. I can't find any specs on the power ( compression force ) of any springs, so I don't know how to compare them.

I used the stock spring on mine with a heavy bolt and buffer, runs like a champ! Recently added a reduced power trigger/hammer spring kit to it because of using pistol primers, it did reduce trigger pull but not all that much.