In Lyman's test on a 2nd Pattern Bess, they achieve higher velocities from 2F and 3F, however the 3F created much higher pressures. I assume this same phenominum might be happening with the 3F Pyrodex. The 'extra' pressure may be causing the crossing. I suspect 120gr. 3F Pyro is developing more pressure than your 180gr. charge of 2F. I use 2F BP in guns over .45 cal. and 2F or 1F in all ctg. guns. 3F has never shot as accurately. 1F gives the best accuracy, normally.
: I've used 1F in a .50x3-1/4" but the velocities were inferior to the same charge of 2F. I am talking of real BP here, not the replacements. In the .50 ctg. rifle, 140gr. charge of 3F ruptured the primer due to excessive pressure. W.W.Greener did much the same with a .450 BP Express with 'fine' rifle powder except the gun was ruined. This was only a 120gr. charge with 340gr. bullets. Do not take BP pressures for granted. If 140gr.2F delievers 28,000 PSI with a 550gr. bullet, switching powder granulation to finer grade will definitey raise pressure to unsafe levels. Makes me question the safety all the 150gr. charges I hear of in the modern rifles that load from the muzzle- in smaller dia barrels.
: I've never found Pyrodex to shoot as accurately as BP, even though Pyrodex is much cleaner burning.
: SingleF GOEX BP might give good results in the double rifle. You'll just have to try it. I expect if the barrel length is long enough, 1F might give higher velcoities than the smaller granulations.(as in a 42 to 46" Bess barrel)
: If you have the old BP handbook by Lyman, check pressure levels of .50, .54 and .58 cal rifles at a given velocity. You will see a co-relating pressure level to achieve these velocities. This is a good rule of thumb. For example, each calibre developes about the same pressure for (example)1,700fps, regardless of it's bore size with the expection that the larger the bore, the pressure developed at that velocity is slightly less than the smaller bores (given the same BP granulation). I am referring to round balls here.