I once believed that all there was to recoil was the physics absolutes of kinetic energy, i.e., the ejecta expelled from the barrel at a given velocity imparted a rearward velocity to a gun of a given weight. One half mass times velocity squared and you had the recoil energy. I still mostly believe that, but I really think there's got to be more to it.
I encourage all here to read the article "Something For Nothing" in the Jan. 2010 American Rifleman. It tells of a new powder used in Hornady Superformance ammo that give measurably greater velocity with less recoil. After you read the article it sounds plausible though I have not tried it, and anyway, I only have my unscientific shoulder to evaluate it.
I have burned a LOT of black powder, but very little in shotguns. For the most part, I would characterize the recoil as more of a shove than a sharp jab. A .69 cal Zoave replica I had, with 160 grs. of FFG and a 440 gr. ball would rear up and push me back a couple steps. But it never hurt a bit. I'm going to relate a story that I have been reluctant to tell when we've discussed this recoil thing in the past... partially because it is a bit long and mostly because I have no explanation for it. I'll try to keep it short.
About 5 yrs. ago, I decided to try a new bullet in my .50 cal Hawken flintlock. This was the T/C Maxi Hunter 325 gr., a pure lead hollow point. I had been using the 370 gr. Maxi Ball, and thought this new bullet might shoot a bit flatter being 45 grs. lighter, and hopefully it would expand better being a hollow point. I bought 2 boxes of 20 and hoped they would shoot near the same point of impact as the heavier Maxi Ball with the same charge of 90 gr. of GOEX FFG.
The first shot hit center and about an inch higher than the Maxi Balls. Second shot almost touched. "Cool," I thought, "one more like that and it's time to go hunting." I fired and that curved brass buttplate and thin comb smacked me like a sledgehammer. I thought maybe I screwed up and double charged or rammed down two bullets. A count of bullets and pre-measured powder vials proved I had not made a mistake, and no, I had not shot my ramrod. The shot went several inches high and wide.
I loaded another and put the bullet right in with the first two. With still no idea what happened on the hard recoiling shot, I decided to try one more. Again, it kicked like a mule and the shot went high and wide. Now, I'd been playing this flintlock game for over 25 years and never ever had anything like this happen.
I kept shooting and would get a normal recoiling shot or two that went where I expected it to go, then I'd get one that literally belted me and sent the bullet high and wide by 7 to 10 inches. The difference in recoil was huge. Instead of the normal hard shove that never bothered me, this was like shooting a 12 ga. magnum slug gun with a curved brass buttplate. It HURT, and I've never been particularly recoil sensitive. Then I noticed that the hard kickers were harder to ram down the barrel, so it became predictable that I was about to get walloped. Even so, I tried my best to avoid flinching, and with the set trigger and sandbag rests, I think I did. Still, those hard kickers flew wild on the target. I was losing daylight and decided to quit for the day, clean the gun, and try again the next day at a makeshift range near the hunting camp.
The next morning, I hunted grouse until noon and went back to the truck for lunch and to resume sighting in the flintlock for deer season which opened in 4 days. Well, it was the same as the previous day with those Maxi Hunter bullets. Roughly 1/2 of them kicked and shot normally while the other 1/2 felt like Mike Tyson was using brass knuckles on my poor shoulder. Part way into the second box of 325 gr. Maxi Hunters, I decided this was no fun any more. I shot several of the old 370 Maxi Balls I normally used to confirm that they and the gun were OK. No problem, except that my shoulder and cheek bone were already beaten to a pulp, so it was hard not to flinch.
I put the flintlock back in the truck and went back out grouse hunting for the rest of the day. I secretly hoped I would not flush any birds because I did not wish to fire my light 16 ga. DS Lefever double. (now we're on topic)
Now I'll concede that firing a 370 gr. conical with 90 gr. of FFG may give more than a little shove, but it isn't at all bad. So explain to me how the same 90 gr. charge with a 45 gr. lighter bullet, both pure lead, could kick dramatically more. But only sometimes.
Recalling that the hard kicking shots seemed harder to ram down the bore, I miked the remaining 10 or 12 Maxi Hunters. As I recall, some were around .495" like the Maxi Balls, and some ran as much as .003"-.004" larger. But still, were talking pure lead projectiles here so I would not think that could account for the vast difference in recoil. Same lube. Same loading technique that never gave me a problem over decades and thousands of shots. Same powder charge from the same can of FFG Goex. So how in hell do I account for the huge difference in recoil? If I had to quantify it, I would have to say the heavy kicks were at least twice as hard as the normal kicks. It was enough that I was concerned I might split the stock. Those of you who are rolling your eyes are justified. I have no explanation.
