CGN user 2CentTactical posted this link a while back, and I've never seen any other reference to it. If I understand it correctly, it solves the last round bolt hold open problem by shimming under the takedown screw immediately in front of the magazine well.
Has anyone tried this on one of the problem rifles? If it works, it seems like a better solution than only being able to use certain types of magazines, or requiring extra power mag springs.
https://gunmagwarehouse.com/blog/ruger-pc-carbine-fixing-it-for-glock-mags/
Relevant text copied here:
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Ruger PC Carbine: fixing it for Glock mags
Posted on April 30, 2020 • by Ryan Houtekamer
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Glock Magazine Carbine: minor issue, simple solution
... the Glock magwell did not want to play well with others. Though I’d read no reports of the SR9 magazine well causing issues, the same could not be said for its alternate. In this case the last round bolt hold open did not function. It took a lot of consideration, frequent scratching of heads, and well-delivered profanity to come up with a solution.
The problem, as best I can tell, was likely due to tolerance stacking. This comes from the wide size range of each part; once you hit the extreme ends it can cause issues. When the forward bolt is torqued down the bolt hold open no longer functions (or that what’s happened in this rifle). This seems to have been caused by flex in the stock, binding the arm that lifts up and locks the bolt back when the last round is fired.
In my case, the fix was very simple. I just used a tiny piece of cheap, clear plastic clamshell packaging — you know, the stuff that should open right up but really presents enough of a fight that you wind up manhandling it until it tears or just cutting it open so your prize drops out.
I cut a small (roughly 1 in.) square hole, drilled a hole in it the size of the forward bolt, and used it as a washer.
I chose to go with plastic because it is somewhat compressible under the torque of the bolt and because it could be made wider than a washer (thus spreading out the load).
This allowed me to fully torque each bolt down and now the bolt hold-open works exactly as it should.
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