I haven't a clue what the difference in trajectory is with a small bore, ie: 20. I have a program that will tell me, though, as many people do.
The reason someone might use 120gr. in a 20 bore over 100gr. or 90gr. or 80gr. charge is not only the few to 5" inches flatter shooting trajectory over 125yards, but MOST importantly, the improved accuracy with the heavier load in typical slower twist rifle barrels.
The newer Forsyth pattern slow twists, the the twist rate than James himself recommended for shooting dangerous game in dangerous game rifles, appears to be too slow for the small bores, requiring too much powder to shoot well.
Forsyth never suggested these twists for less than a 16 bore, ie: .662". Dan's own testing has shown that an 80 to 85" twist is all that is necessary to give good range and accuracy & velocity with that bore size. To slow the twist even more and in a small bore as well, was a mistake, I feel.
The difference between a .562" ball starting at an 80gr. load of about 1,200fps (was inaccurate in my 56" twist .58Dbl.Rfl), compared to my accuracy load of 110gr. at 1,450fps, is approximately 4" higher trajectory at 125yards - add that to the VASTLY improved accuracy of the heavier load, 2-3" compared to 9"-10" and it does make a difference. A HUGE difference - especially when shooting at deer. Add the 4" extra trajectory to the inaccuracy of 9-10"group and that 13" to 14" becomes excessive. Might be lucky and hit the centre or upper quadrant of the group, or simply shoot low or left or right and cripple, gut shoot or miss. If it can go wrong, it generally will - Murphy's law too often rears it's head.
For someone who uses load combinations that go to $#*! with increased charges and velocities, then there is no reason to use heavier charges. You need accuracy to hit WHERE you need to hit. Use the most accurate load and adjust the GAME hunted and the MAX RANGE to suit your restrictions.
I've seen guys print that their .50 or .54's accuracy goes all to -ell if they use more than 70gr. of powder. I've never seen that, but I've seen it in print & I've heard guys say it. Invariably, they are shooting wimpy .010"patches that burn out with decent powder charges that shoot perfectly in tight combinations in the same barrels. If that is their limit, then deer at quite close range are also their limit. Don't bother going after moose or elk.
When using slippery lubes, ie: hunting lubes, I've found All of my rifles except for the .69,. deliver better accuracy with much increased charges over the water-based target-type lubes. The .69 doesn't care as it shoots the same with grease, oil or water and it's accuracy load is currently 140gr. new GOEX. It used to be 165gr. of the older GOEX powder. My second leaf used to be a 200yard zero with 165gr. but 165 of the new powder gives a 200 METER (220yd.) zero - running some 6" high at 200yards. Not enough to screw up a shot on a moose or elk, but enough for sure, on a deer. That is why the laser range finder accompanies me on EVERY hunt. To 125yards, flat sight, dead deer, moose, elk or bear. After that, one uses his leaf sights, 150 and 200.
If indeed that new GOEX 140gr. 2F .69 load is producing 1,500 to 1,550fps, I can rest assured it is producing the same breech pressure as 1,500fps produces with 2F powder in a .54, or a .58 or a .62. & it isn't much. It is also the same pressure that was produced before when 165gr. of powder was needed for the same velocity. Many equate recoil with pressure - indeed, as you increase the charge in any gun, the pressure and recoil go up. But I can tell you my buddy's .75 with 100 to 110gr. of powder (1,150fps) produces a LOT more recoil than 140gr. does in my .69. His ball weight is 600gr. compared to 480gr. for mine, yes his pressure is very minimal & much less than mine is.
Lyman's book also shows us this relationship between velocities and pressure with "like" powders. Too- in my rifle, it takes 200gr. of 2F to produce 1,700fps. The recoil is quite heavy - about 55 to 60 fpe, but - the actual pressure is similar to the same as a 1,700fps load in a .54 - at what? 115 to 120 modern 2F to get that speed? Is that 200gr. charge a hunting load? H--l no- kicks too much for me, but the pressure is not excessive for a good rifle, just as 1,700fps is not excessive for a good .54 or .58. It is merely an example of velocity and pressure - as the calibre gets larger, the pressure per powder charge drops, just as the velocity does. It takes more of one to get the other.
I should note here, than stock damage can and does happen with high recoiling loads. Forsyth said to avoid unnecessarily "shaking the stock".
Incidentally, my buddy Keith started with 200gr. in that 11 bore rifle Taylor built him and with a hardened ball, ie: WW alloy, he could not keep one inside a moose. It shot right through them, most dropping at the shot - boom/thump. Recoil was horrific though. His velocity was right around 1,500 to 1,550fps. He kept reducing the charge until he was able to recovere the WW alloy round balls form the moose he and his family shot with that rifle. It rather outclassed their TC's - HA! That happened at around 125gr. and 1,200fps. Much easier to shoot and about the same recoil as my 14 bore rifle with 140gr. load produced- so - quite manageable.
That 125gr. load has probably killed over 20 moose over the years since 1987 - works for him, with the Scope he had Taylor install. It shot in the 3" range, at 100yards. More powder shot better, but with the brush in our normal moose area, 125 yards was a good, easy range.
For someone who shoots game right off the muzzle, ie: less than 100yards, trajectories mean little, unless shooting light loads with elongated projectiles - then the difference in 100yard trajectories start to matter.
A deer has about a 7" to 8" kill zone, while a bull moose has about 30" and an elk, can be fairly close to that- 28", maybe down to 24"- depending on size.
ref:(was inaccurate in my .58DR)- made individual 2" groups, but the barrels printed 6" apart. With 110gr. 2F, the gun produced individual 1/2" groups at 50 yards, and 1" combined, 3 rights and 3 lefts - thus, 110gr. was THE load for this rifle for target shooting. For hunting with an oil or greased patch, it took 125gr. of 2F to make the barrels regulate, ie: shoot together. 110gr., the accuracy load with water based lube, shot as poorly as the 80gr. had originally with water lube, with individual barrel groups some 6" spread at 50yards - totally useless for a double rifle for hunting to 125 yard or even further ranges. 80gr. with the oiled hunting patch, made patterns all over a 8x11" target. Horrible! For that rifle, less than 125gr. 2F was useless.
In my opinion, 3F is for pistols and squirrel rifles, .25 to .36, & has use up to .45 calibre. In .40 and .45, I achieved the same accuracy at lower pressures using 2F. I do use 3F in my .40 and .45, however only if I have lots of it. 2F is the only powder I used in my first .50, 4 different .58's, my .62 smoothbore and my current .69 rifle.
