June 29, 2007, 22:35
Ok, have an Imbel barrel and bolt, DSA receiver and DSA para carrier. I am not sure which measurement on the locking shoulder is correct, but here are the 2 measurements from the shoulder I have: 0.260" on the short side, and 0.265" on the long side. The bolt will drop in about 3/4 of the way with the GO gauge in, and I can push it all the way in. With the NO GO gauge in place, the bolt just starts to drop in, but I can hammer in in with a plastic hammer. I know I need a smaller locking shoulder, but how do I figure out what I need?
June 29, 2007, 22:46
Measure the LS on the same axis as the "foot". It should close with just light pressure on the GO. You need to get a set of pin gauges and do it right -- with the extractor removed.
June 29, 2007, 23:11
1. Actually you will need a LARGER locking shoulder.
2. What you are doing with just GO and NO-GO gauges and the locking shoulder installed in the receiver, no pin gages, will simply tell you if your existing LS is too small or too big, but not the proper LS size.
3. Doing this with the LS already installed in the receiver means you're just going to need to pound it out again to insert pin gages and when installing the correct LS. This can open up the LS hole in the receiver, a bad thing. Always keep the LS r&r to a minimum.
4. Shootist is saying to measure the locking shoulder from the "front to back" dimension as it is installed in the rifle. The "foot" is that little arm thingy that sticks out from the right side of the LS.
5. If you are getting a .005" variance, either something is wrong with your measuring technique or your tools or your locking shoulder.
6. Very important to remove the extractor.
7. Very important to INCLUDE THE BOLT CARRIER.
8. Very important to make sure EVERYTHING is clean, clean, clean.
9. Insert a pin gauge into the locking shoulder hole, insert GO headspace gauge into the chamber, try to close bolt/carrier (with extractor removed). No fit, use a smaller pin; fit, try a larger pin.
10. Measuring is finished when you have found a pin that allows you to close the bolt/carrier with firm pressure, not too firm, you aren't trying to crush the thing in there or hammer it in place.
11. Example with 1.630" Forster GO gauge: Final pin is .261"; for standard 1.632" headspace, SUBTRACT .001" from the pin size, you will need a .260" LS (with setback after initial firing, your headspace will settle into about 1.6315" or so). For shorter 1.630" headspace, use the same LS as pin size, i.e. .261" (with setback after initial firing, your headspace will settle into about 1.6305" or so). Note that the tighter the headspace, the more likelihood of chambering problems particularly with commercial rounds.
12. Example with 1.632" Clymer GO gauge: Final pin is .261"; for standard 1.632" headspace, use the same LS as pin size, i.e. .261" (with setback after initial firing, your headspace will settle into about 1.6325" or so). For shorter 1.630" headspace, ADD .002" to the pin size, i.e. you will need a .263" LS (with setback after initial firing, your headspace will settle into about 1.6305" or so). Note that the tighter the headspace, the more likelihood of chambering problems particularly with commercial rounds.
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