HELP with Erratic bolt hold open with JP SCS

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Hoosiershooter
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HELP with Erratic bolt hold open with JP SCS

Post by Hoosiershooter »

Started getting erratic last shot bolt hold open, more with supers than subs. After swapping lots of parts including buffers, weights on the Silent Capture Spring, lowers and bolts, the common denominator seems to be the SCS. Then I noticed that with the SCS in any of my guns, the bolt does not travel as far to the rear as it does with a standard buffer and spring. In fact, in all my lower receivers,when the bolt goes to the rear, it barely clears the catch. I figure with supers the bolt is bouncing off of the delrin pad in the rear so fast and hard that it is not t able to capture it.

It looks like the carrier makes just a little nice contact with the buffer when i close the gun. The compression travel is 3.75 inches which compares to the specs I've seen in other threads.

The standard buffers seem to clear the catch by about 7/64 on my DPMS factory carbine, 3/16 on Colt m-16, 11/64 on DPMS carbine buffer in the spikes lower on the 300BLK. The SCS is less than 1/16 inch all measured by sticking drill bits in the gap. I'm thinking about shortening the delrin bumper by about .05 inch.

Has anybody else seen problems like this? Don't know if its a parts compatibility issue or an out of spec SCS.
Last edited by Hoosiershooter on Sat Apr 20, 2019 10:08 pm, edited 2 times in total.
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dellet
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Re: HELP with Erratic bolt hold open with JP SCS

Post by dellet »

I’ve never had the problem and shoot some pretty fast supers.

What you might try is to back off the tube one turn, that will equal .0625”
300 Blackout, not just for sub-sonics.
Hoosiershooter
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Re: HELP with Erratic bolt hold open with JP SCS

Post by Hoosiershooter »

Re: HELP with Erratic bolt hold open with JP SCS
Post by dellet » Sat Apr 20, 2019 6:57 pm

I’ve never had the problem and shoot some pretty fast supers.

What you might try is to back off the tube one turn, that will equal .0625”

Thanks dellet, I tried that but then I could here the SCS rattling to and fro behind the bolt. I didn't feel like that would be a workable enough option to even try to shoot it. I edited my original post to indicate the wide rang of tolerances i see between guns. The SCS allows the smalletst gap in all my guns.
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dellet
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Re: HELP with Erratic bolt hold open with JP SCS

Post by dellet »

Something about that does not make sense, when you turn it back one turn, does it quit reaching the buffer stop pin?
Or are you not using it?

I also realized that I lied to you. I have had a similar problem. Mine turned out to be a PMag issue. The follower was so sloppy on the body that it would miss the bolt catch completey. If it locked back at all it was on the carrier not the bolt face.

Have also heard of the catch itself being short.

I did check mine and they are all between 7/64 loose fit, and 1/8 mpossible.

Did you fire the others with the SCS or just check clearance.
300 Blackout, not just for sub-sonics.
Hoosiershooter
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Re: HELP with Erratic bolt hold open with JP SCS

Post by Hoosiershooter »

I'm not using the buffer stop pin.

I used P-mags and D&H aluminum mags, both with green followers. To my surprise, the p-mags work a little better but not 100%. I suspect however that the system is working marginally and am looking for optimal operating zone. The low clearance between the bolt catch and bolt is my concern now even if I found mags that do make the system work.

So far it seems like whatever weapon I put the SCS into the gap is reduced to barely clearing the catch.

I suppose ill do a little more experimenting before trimming on the delrin

I guess I'm trying to figure out if there would be any negative consequences to shortening the delrin bumper. Hopefully i can get a replacement from JP if i screw it up.

I think I need ti get the gap opened up to get it operating in the middle of the sweet zone for 100% function.
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dellet
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Re: HELP with Erratic bolt hold open with JP SCS

Post by dellet »

Just pulled one to measure.

7 1/6” overall.
3 1/4” compressed
3 3/4” + compression

The compressed length and amount of compression was hard to measure and as you see the numbers don’t add up. I measured the distance from base of the weight to top of the outside diameter of delrin. The top of the Delrin is concave towards the center.

Maybe that helps.
300 Blackout, not just for sub-sonics.
Hoosiershooter
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Re: HELP with Erratic bolt hold open with JP SCS

Post by Hoosiershooter »

I tried turning the buffer tube out again 1 turn. Maybe I turned it out 2 turns the first time producing buffer rattle. 1 turn out did seem to keep the buffer barely touching the bolt carrier and gave me an acceptable 7/64 gap between the catch and fully retracted bolt.

I just don't get all the variation I see in the gaps,and why the reduced gap seems to follow the SCS from one gun to another. Since the m-16 mill spec has largest gap I'm wondering if the commercial carbine tubes have wider tolerances.
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