Bob, there's no question subs generate more pressure & as you point out, likely have a very different pressure curve. The 70 gr equiv load I tried of 3f H777 seems very similar to 70 gr of real bp as the point of impact didn't change, but in my Ruger Old Army revolver, I have chronographed a substantial difference in velocity which has to translate to higher pressure. Maybe the vent in my rifle helps equalize the pressure? Maybe E. Ogre can pipe in here & offer information on pressure curves with subs? I need to take the Chrony out next time I shoot subs in my rifle & test for any variances in velocity between the 2 powders.
When this 777 first came out and I took it apart and identified what was in it I had questions on possible pressures. Beyond any question Hodgdon looked at this closely during the experimental batches leading up to the final product.
I noted that in my mule ear .50 cal round ball rifle the 777 came close to the then available Swiss powder. That told me that the 777 produced pressures close to those seen in the Swiss powder. Nothing to get excited about in most ml guns.
The only thing that rattled me was if these "strong" powders are used in one of the Spanish-made in-lines there could be problems. But in our ml guns with the fixed breech plugs I could see no danger.
Hodgdon put a lot of thought into this 777 powder. When I first tested it I remarked that if bp were to become unavailable I would switch flinters over to percussion and shoot the 777 in them. Good velocities even in reduced charges and easy cleanup. But if one gets carried away with the charge sizes one may expect to see some baked on fouling in the bore ahead of where the projectile sat on the charge. Which I saw in some of my Swiss powder testing. If you get powder combustion gas temperatures up close to the melting point of potassium carbonate (in the residue) you will see this fusion and sticking to the bore walls. Then you simply back off a bit on the charge size.
In my mule ear lock GPR the 777 worked almost perfectly. The side lock Trade Rifle was another matter entirely when it came to reliable ignition.
When the in-lines firing plastic saboted pistol bullets started to really sell Pyrodex presented problems in them. I watched this at the gun club one evening when shooting my flinter with Hilljack. Guy and his wife showed up with two brand new Knight "disk" in-line rifles. They came with a box of Pyrodex pellets and a bag of disks holding 209 shotgun primers. Nothing to clean the bore's with. So the guy loads up with two pellets and fires the rifle. Very accurate. Loads and fires a second shot. When he went to load the third plastic sabot it would not seat down on the pellets. He had to pull the breech plug and drive the sabot out the muzzle. So I loaned him some cleaning patches and my bottle of Lehigh lube. He then repreated the two shots with the third stuck.
Now the punch line to this was as he was going through this he was lecturing Hilljack and I on the evils of dirty black powder and unreliable flintlocks. For the whole time! Sounded like a stuck record! The funny part is that he was at the table next to us and his problems cut Hilljack's rate of fire with my .45 caliber flinter.
When I first looked at the 777 and took it apart it was clear to me that they removed most, if not all, of the potassium perchlorate found in Pyrodex to get rid of the little crysals of potassium chloride found after firing it. The problem with the plastic sabots was that as you pushed one down the bore the little crystals would be forced into the surfaces of the sabots to a point where you simply could not push them any further down the bore.
Then compared to Pyrodex they increased the ballistic strength to get velocities up. The in-line crowd expected near smokeless velocities. Without the smokeless pressure surge.
When 777 first came on the market they stated that it was intended for in-line ml rifles. They did not recommend that it be used in sidelock guns. Now we see that evolved to where it is suggested that the breech of a flintlock first be primed with a little black powder and then black powder in the pan. The breech prime of BP working like the little bag of bp in large-caliber artillery loadings as an "intermediate primer".
E. Ogre
