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I recently got a new PC Carbine 9mm. It's a simple gun to take down, put together but the Barrel Adjustment ring has some quirks to it to understand for best accuracy.

The Ruger manual states -

2) Install the barrel/forend assembly by inserting it into the receiver as shown in
Figure 43, and rotating it clockwise until it locks into place.
3) Tighten the adjustment knob by rotating it clockwise until the barrel/forend
assembly is securely connected.
NOTE: You should feel some resistance when turning the barrel/forend
assembly into the receiver, but it should not be difficult. If further adjustment
is needed, remove the barrel/forend
Here is what I found, if you adjust the adjustment ring tight as you can get it while the barrel is assembled and "call it good", there will still be a slight play- you can verify the play by holding the receiver with one hand and pushing side to side on the muzzle.. you will likely feel a very slight rocking. That rocking is not good for accuracy. Any rocking will be changing the point of impact throwing off aim. In fact, you may see this when shooting when suddenly a tight grouping will shift for no apparent reason and then reform a new grouping offset to the first.. THAT is a result of that "rocking" that the barrel has has shifted relative to receiver.

So here is the solution. If you ONLY tighten the adjustment nut hand tight while together, that will not be as tight as it can go. First, tighten it by hand together- THEN take the barrel off and THEN tighten adjustment rings several clicks and then reinstall barrel and test for snugness- rinse repeat. Eventually you get a resistance to rotating the barrel on and find the wobbling and rocking is completely gone even when you really push on the barrel. It's interesting to note that the differences between a barrel that is too tight to install, has some slight resistance, and not as tight as it could be but fully finger tight while on- is only about 4 clicks difference on the adjustment nut.

I suspect in reading different PC carbine shooters this may explain the large variance where some owners are getting 2MOA and others are struggling with 6MOA and worse. Though there are a few that had defective barrels.


Hope this helps.
 

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I “ve had a pc carbine about a year now and it has never shot straight.I adjust the lock ring every time out.A month ago, by accident I realized that the barrel had excess play on the receiver end of the barrel. The barrel has side to side play and no lock ring adjustment fixes it.Has anyone else has this problem?
 

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I found the same thing with my PC. I had to take up the slack with the barrel off.
 
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