Sorry not to be around much. Work has kept me tied up lately.
Thermal expansion of a .357 bore by raising its temperature to scalding hot will only be about 0.0002” in diameter. That’s not enough difference for a 0.0010” oversize bullet to start having gas bypass it.
The Nitro 100 NF does sound similar to Tightgroup. Nitro 100 NF (for New Formula) replaced the old Nitro 100 which Accurate could not get made any longer. The NF was wrung out in shotguns first, with an early warning by Accurate not to use it in metallic cartridge loads until they had some load testing done and published. Now they've begun to get that data together, and
the FAQ says to use 10% less Nitro 100 NF than of the old Nitro 100 as a starting point. This would mean starting with starting loads 10% lower than the old Nitro 100 starting loads, and working up from there.
N310 and Hodgdon Clays are both very fast, too, and Clays is also very clean (haven’t yet tried the 2 lb bottle of N310 I have in the cellar), so I suspect you’ll find the same issue with both of them. I suspect the issue is mainly dependent on burn rate.
Normally, in a rifle or in a pistol burning slow magnum powders, when you increase barrel friction, velocity goes up. Put lubricated moly bullets in a gun and velocity goes down. This occurs because the powders involved are slow enough that the added resistance increases their rate of burn and pressure rise, increasing peak pressure and completeness of burn in the bore by making them behave like faster burning powders. (Most folks assume the relative burn rates of powders on a chart are constant, but in fact they only hold up under one set of standard conditions, and at different pressures and temperatures the order changes some.) In effect, the added resistance provides increased confinement and resistance to expansion of the volume the powder is burning in. Lubrication has the opposite effect.
But with an extremely fast, low progressivity (or even digressive burning) powders, expansion by the time the pressure peak is reached is small. That means you can’t use much charge weight without making so much gas in that still-small expanded space that pressure becomes excessive. So you make a small, fixed quantity of gas with these powders that are often done burning and at their peak pressure before the bullet base has cleared the case mouth, much less having passed through the throat. When the bullet enters the bore under this circumstance, the effect of friction turns around. It cannot increase pressure the way it would with a .357 case full of 296. Instead, with all gas pressure already made, bore friction just opposes the force generated by that pressure. Moreover, in a revolver added resistance slowing the bullet means more time for a greater percentage of the limited gas quantity to bleed down through the barrel/cylinder gap. That lowers pressure still further. So there’s an exponential drop in velocity as the bore friction increases.
With lubricated lead bullets, the lubricant limits how fast fouling builds up, but also the alloys used are more slippery against steel in the first place and are softer and squeeze through constrictions more easily, so you see a slower buildup of friction with fouling. Copper bullets won’t have that advantage.
So, what can you do about all this. Several things are possible to do. Anything you can do to reduce the rate of fouling build up will reduce the rate of velocity drop. Polishing the bore surface helps slow buildup for lead, copper, or gilding metal. Increasing either bore or bullet lubrication helps. There are several approaches to both. Firelapping by any of several variation of method will significantly smooth the bore and tend to remove constrictions or other imperfections. You could just soak three dozen cast bullets for a few days in mineral spirits so and old toothbrush can clean the lube grooves, then replace that lube with a lapping compound or even just with JB Bore paste to polish rather than actually lap much metal out. Fire them slow, as you are now. Frequent cleaning (after every cylinder full) is important to prevent fouling from masking part of the bore from the polishing action.
You can also apply lubricants to a bore that will last a good while at your pressures. Clean the barrel down to bare metal (should be done before firelapping, too; search past posts on cleaning for ways to do this) and plug the muzzle and fill it to the forcing cone with Sprinco’s Plate+ Silver. Leave it 72 hours at room temperature and put the liquid back in the bottle (it’s not measurably depleted by this application). This leave the bore with a semi-permanent lube that includes micronize acid-neutralized moly that causes rifle bullets to lose about the same velocity they do when shooting moly-plated bullets. It lasts about 1000 rounds in a rifle if you don’t refresh the application, but if you by patch application after every cleaning session, the coating is maintained.
Shooter’s Solutions has a product called Moly-fusion that will also put a long lasting coating in the bore.
You could try applying a lubricant to the bullets directly. I’ve not played with the plated bullets much, but I expect a thin application of thinned Lee Liquid Alox at these low load pressures would not create any pressure issues. You could try powder coating them with plastic resin as is described in detail in the cast bullet forum. These lubes should help the bullet swage into the forcing cone more easily despite the slight added thickness. Many revolvers seem to shoot best with bullets fattened to the full diameter of their cylinder throats anyway.
. And they're excellent shooters (aside from the copper buildup thing). The best thing about them is that they cut extremely sharp, full-diameter circles through cardboard- they cut sharper than lead DEWC's. So I'll probably shoot them intermixed with lead DEWC's at the matches.