OK, look at it this way. A typical AA12ga 1 1/8 oz load at 1200fps, what's that run, 8200psi? Then per Bell, add 10% for chamber length less than nominal 2 3/4" and = 9020, right? So you know from the p. 515 chart that your gun, assuming that it is not dangerously thin walled, off face or stock oill soaked, cracked, etc, has a recommended service load average of 9500 and a mean definitive proof load of 13,700psi. So are you good to go?
Then consider a typical Win AA 1 oz load at expended hull length of 2 5/8. That's 1 /8" of very thin plastic wall over the nominal chamber length. Is there anyone who would seriously suggest that that sliver would cause an increase of even 10%?
If you had an abundance of caution, you would never shoot an old gun.....at least that's what the manufacturers would tell you. So, if you do choose to accept some small risk, you can either try to understand these load and pressure matters, because some really get into this and like to know, or if you you don't want to think about it, just call RST and order a case because they formulate for older guns.
I am with Pete, I really like light loads at targets. But I also am somewhat of a student of shotgun ballistics and like to know what I can load to and sometimes I take real delight in dusting the 60 and 70 yard targets or the long dove.
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