Shooting an action/barrel loose?

[PineyCreek22]

If any of the members understand metalurgy I need your help.  As I understand it  there is probably 2 ways to loosen an action.  1 is to set the barrel lug back and 2 is to stress the frame beyond its capabilities.  Also as I understand it there are 2 primary properties of steel that are involved here, yield strength and tensile strength. 


In the case of a SB2 receiver (all these numbers are half vast guesses on my part) we assume the steel is 4041 heat treated or similar with a ball park tensile strength of 120,000 lbs/sq" and a yield strength of 90,000 lbs/sq".   I am guessing that the cross sectional area of the receiver sides are about 3/8 sq"  this translates to an ability of the receiver to handle no more than 45,000 psi back thrust.  Since this thrust is against the breech face and also against the barrel pin would this force be doubled? So in the case of a 30/30 factory round be about 17,000 lbs/sq".


In the case of the barrel lug weld I assume in the case of the 30/30 the force would be about 8,500 lbs/sq".


I used back thrust of 8,500 lbs from a 30/30 but in the case of a .270 these values would be increased by a factor of almost 2.


If the tensile and yield properties of the metal involved are  exceeded one time does this weaken the metal and reduce its ability to withstand these forces the second and third time and so on?


Obviously it does, otherwise our receivers and barrels wouldn't loosen up.  So within what range should we keep loads so that  they don't loosen up?  Piney Creek

[quickdtoo]

Underlug setback is due to the small surface area of the pivot cut out, if it was solid it would be more serviceable, see Underlug Improvement in the FAQs sticky, I've made this improvement on my 500SW barrel and on one of my 270 Win barrels as well as the 300WSM test barrel, no issues with any of them, the 500S&W barrel has over 250 full pressure rounds thru it and has since been rechambered to 499HE, still working fine.

As for SB2 frame "stretch" I had a 300WSM Handi barrel(rechambered 308Win) that the previous owner shot 100 factory rounds thru with no issues to his frame, it wasn't accurate due to the internal action flex, but the frame was undamaged. I bought the barrel from him for the cost of shipping and tried working up midlevel handloads, the larger .555" case head was just too much for it, 150gr @ 3000fps was the best it would do accurately, warmer groups opened up, the large case head of the short mags is why no one makes short mag barrels for TC Encores any more. The original barrel that was fitted to the frame that I used for the 300WSM testing still locked up tight after the testing, so I know for a fact the frame wasn't "stretched", but the pin that anchors the locking latch on the H&R flexes under pressure and gives the impression that the frame is stretching and allows the barrel to partially unbreech leaving a couple thousandths gap between barrel and standing breech, this is also what stretches the brass and the reason brass in full pressure high pressure bottle-necked chamberings on break-barrels has to be full length resized, not just neck sized. Barrel underlugs do setback tho, that's where loose barrels come from, not the frame stretching that everyone "thinks" is happening.

Tim

EDIT: Fred touches on this subject on his 257 Roberts Ultra which setback after 160 rounds. Another issue is just plain wear of the pin and pivot, one member had his 38-55 Target setback after about 3000 rounds IIRC, all cast bullet loads.

http://www.angelfire.com/ma/ZERMEL/gapfp.html

[PineyCreek22]

Thanks Tim for the information you shared and the link, all very informative.  Piney Creel