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Sounds like a stabilization issue to me.
Faster twist may not stabilize lighter bullets properly, and 1x9" is on the edge for the 55s. You should not shoot the M-855/SS-109s in a 1x12" for that very reason, and the 1x7" is prone to doing just what you described with M-193 ammo. 5% does not sound terrible, for a blaster. If he wants to find out, switch to a 62-69 grain bullet and try it. Did it shoot the 55s before without any tumbling?
If you suspect a flaw in the bore, get a borescope and check it out.
TS is right, if it has been repeatedly shot to a color change, it could be burned out.
Also, if it is chrome lined, the lining may be shooting out.
Not sure that I would want to chop a 20" to 16" because of cycle and timing issues, but that would be a function issue, not a stabilization issue.
Best of luck.
TR
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"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat." - President Theodore Roosevelt, 1910
De Oppresso Liber 01/20/2025
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