Minor issue with my pistol lower

[titanse05]

Well I just finished putting together a future SBR that is running a KAK pistol buffer tube, buffer spring for collapsible stocks from Rainier Arms and an H buffer. When I pull the charging handle back the BCG doesn't lock back as it should. I suspect that the spring is the issue as the ones I have got from RA seem to be running long.

Any suggestions to what else could be causing this?

[DeviantLogic]

Bolt catch?

[titanse05]

Bolt catch is a proprietary piece that works in conjunction with the right side bolt release but I don't think it's the problem. The BCG isn't even coming close to getting back far enough to engage the catch.

[cjd3]

I've never built a pistol lower, however, I didn't think that the lowers need a different buffer or spring. Is the charging handle catching on the upper? How far from catch is the bolt? Pics?

[titanse05]

About 1/2" from bolt catch.

[RiverRat]

Questions

Are you SURE thats an H buffer?

Is the action spring 10 1/16 - 11 1/4 inch in length?

Is there a 9mm spacer plug stuck in the back of that receiver extension?

Does your fire control group pass a function test?

[SINNER]

Sounds like you have a rifle spring. Swap out with a known working carbine spring.

[Eric]

Sounds like you have a rifle spring. Swap out with a known working carbine spring. That was my first thought. Standing by for spring measurement...

RIFLE: 11 3/4 Inches (29 85 cm)
minimum to 13 1./2 inches (34 29 cm)
maximum

CARBINE: 10 1/16 inches (25.56 cm)
minimum to 11 1/4 inches (28.58 cm)
maximum.

I've never used a KAK pistol buffer tube, but from what I understand they are designed to use a carbine spring.

[Jerry R]

Okay, I have the complete KAK Kit. Buffer is unmarked and their website states "Standard Carbine Buffer" - it appears to be "H" weight. However, buffer weight should not be a factor on lockback when using the charging handle unless you have a rifle length buffer.

Again, website states "Carbine Length Spring" - my spring measures exactly 10.75 inches. Ejection pattern is between 3 and 4 O'clock - flawless so far - and locks back when using the CH.

Are you pulling the CH and trying to use the bolt catch manually, or pulling with an empty magazine in the well?

[titanse05]

I'm trying to lock back with magazine in place. I'll get some measurements in a little bit. Will also swap a known good spring from another rifle. Hard to make time with my two little ones :)

[titanse05]

Well I found another carbine spring in my spare parts kit and wouldn't you know it, problem solved.

The two springs I got from RA claiming to be for collapsible stocks are 12.75" long.

Going to have to return them and get the right ones. Emailed RA.

[SINNER]

Have to love the easy fixes. I mixed up some springs once and had the same issue.

[Jerry R]

Glad its fixed !