Lower out of spec causing bolt catch failure to engage. Need a solution.

[mrbieler]

I have an older DD lower that has given me sporadic issues over the years with failure to lock back. I blamed cheap ammo and p-mags for a long time, but because it was a rifle I shot little, I never really dove into the issue.

This summer I am taking my son and two of his friends out and I was getting the rifles zero'd in. I decided to find out what was going on with the rifle. Factory DD lower (BCM FCG added later). Gave me issues with failure to lock back with a carbine set up regardless of buffer weight and now with a rifle extension and buffer. Did some upper and lower swapping and everything pointed to the lower as the issue.

I took a look at the lower and the issue is obvious. I'm angry at myself for not spotting this early. I was to fixated on the issue being a short stroke problem.

For local public range use, I use Hexmags as they are a cheap easy 10 round California friendly solution. I have plenty of other mags, but the hex mags don't attract attention.

The lower is dimensionally out of spec or at least on the far end of tolerances and/or the cut for the bolt catch is poor and allows for too much front to back play. The bolt catch measures within .002 of all my other bolt catches (in other rifles or in the spare box).

On the plus side, it looks like GI mags are OK, but my pmags and hex mags are a no go and that's a frustration.

DD lower with hex mag


DD lower with pmag


DD lower with GI mag


My other lowers all look like this:
BCM lower with hex mag


BCM lower with GI mag



Does anyone make a quality catch with a longer nose? I recall a post from a few years ago about someone modifying a RRA 9mm catch, but does heat treat become an issue? I swapped in a Colt bolt catch and a couple of others from my spares and while some are a fraction better, all are still too short for 100% reliability.

Being in California, replacing lowers can be expensive and PITA. All my lowers are from before the rifle registration in 2014 and I want to keep it that way.

[markm]

I'd check to see if this catch isn't simply shorter than another one. I have seen that many years back. Measure another one to see if it isn't the catch. The lower looks to have the notch in the correct spot in the forging.

[mrbieler]

I'd check to see if this catch isn't simply shorter than another one. I have seen that many years back. Measure another one to see if it isn't the catch. The lower looks to have the notch in the correct spot in the forging.

Front to back, the "tooth" that catches the follower is right in line with the others I have. +/- .002 with my micrometer. The thickness at the pivot point is a hair narrower than the spare Colt I tried, but in line with my others. When I put in the Colt catch, it narrowly worked with mags that wouldn't work before, but a good slap on the lower usually knocked it off the follower. I think the part is the correct size.

I think the notch for the catch is both too sloppy and a little too far to the rear of the lower. I don't have a tool to measure that distance properly.

[markm]

You're right. Comparing it to the BCM lower, it does appear that the cut for the catch is slightly more rearward on the DD.

[greybuff]

Because of your issues w/ Kalifonication and lowers, take it to a "good" machine shop and have one made w/a longer nose or have a new nose tig welded to a spare bolt catch and than re-shape it (it can be re-heat treated). Or have a few thousands milled off the front and put a spacer in the rear of the slot (not ideal but could work).

[ssc]

May I suggest that you contact Roger at Forward Controls for his input. He is local to you. Perhaps his ABC/R v2 may help. https://www.forwardcontrolsdesign.com/

Cheers, Steve

[tehpwnag3]

Contact DD for a possible replacement?

[mrbieler]

May I suggest that you contact Roger at Forward Controls for his input. He is local to you. Perhaps his ABC/R v2 may help. https://www.forwardcontrolsdesign.com/

Cheers, Steve

Thanks. I will. He can't be more than 20 minutes from me.

The lower is about 10 years old so I don't think DD will have much to offer (and I don't expect them too). The issue, being in California, is that even if they did offer to replace it, I'd have to go through registration. I'm not a fan of that.

I was over on the Brian Enos forum and found a thread showing how a 9mm catch was modified, but one of the posts at the bottom suggested this poor mans fix was a poorly fitted bandaid at best. Unfortunately, due to the photo-bucket fiasco, some of the pics are gone.

[Duffy]

Give me a call, maybe there's one with the mag follower trip on the long side of the spec. I think it is the receiver though, you've done a thorough job eliminating the bolt catches and mags variables, the only constant here is the receiver.

If that fails, I'd think DD would take care of it, maybe I could get Joe Marler at DD to help. I don't know Marty, but I am friends with the good folks at SOLGW, they might be able to help sort this out.

[mark5pt56]

Thanks. I will. He can't be more than 20 minutes from me.

The lower is about 10 years old so I don't think DD will have much to offer (and I don't expect them too). The issue, being in California, is that even if they did offer to replace it, I'd have to go through registration. I'm not a fan of that.

I was over on the Brian Enos forum and found a thread showing how a 9mm catch was modified, but one of the posts at the bottom suggested this poor mans fix was a poorly fitted bandaid at best. Unfortunately, due to the photo-bucket fiasco, some of the pics are gone.

All that guy did was shorten the catch tab, dimensionally otherwise they are the same.