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im still a novice here, so feel free to correct or ignore this, but is it not just as simple as bullet stability? the rossi has 1:30 twist, if you are firing 158gr projectiles at subsonic velocity, the barrel twist is not fast enough to stabilise them. you would need to go lighter and/or faster. but interesting the deeper seating made a difference, makes sense that getting rid of some of the empty space in the cartridge would make it more consistent burning as cordite says.
You may be on to something Mr Bang. Ive been thinking about the weight of the projectile in relation to the slow rifling twist, but never considered the velocity as a factor. While I've reloaded a lot of bullets and had success with my .308 projects, I'm still a reloading novice myself. There is a nice Lee mould which makes 125 grain flat pointed bullets and I'd like to try some of those. I'd hoped that I might find someone who could spare me a few to try because it would be a bummer to buy the mould and cast a batch of bullets only to find that there is no significant difference in accuracy.
If I buy any more bullet moulds, I'd also like to get the nice electric lead melter pot that Lee makes. I generally use a big nozzle on a gas torch to heat up a pot full of lead, but it is relatively hazardous and it is difficult to hold a steady temperature. I suppose I could get an electric hot plate which would be a step in the right direction.
If/when I get my new Henry .357 I will be interested to see how it performs with my current loads. I'm not absolutely certain, but I think the twist rate is one in sixteen inches - whereas the Rossi's may be only one in thirty inches. And if the Henry works well with my reduced loads, maybe I will devote my Rossi to shooting faster jacketed bullets... unless of course somebody makes me a decent offer for it :)
The wadcutter projectiles, that shot the best of everything I tried, weigh 148 grains... ten grains lighter than my others. This supports what you've suggested.
there are a bunch of online stability calculators you can play around with to see what will be stable and what won't, not sure if this is the best, but its nice and colourful: https://bergerbullets.com/twist-rate-calculator/. haha, well i guess i am a little invested, and quite curious to see whether you can tighten your groups, particularly after the other comments about faulty barrels in some rossi's.... but, i also just opened up some stuff i got from my Granddad's stuff when he passed away recently, and found a couple of old winchester 1873(??) stocks and for-ends, so i might have found a more time-consuming way to satisfy my lever action craving!
Thanks Bang, I will check out that calculator. I'm really glad I've had a decent playtime with the lever action. When I was younger I was a bit envious of folks living in countries where they can freely use handguns. I've since tried a small amount of handgun shooting and I guess I got it out of my system to some extent. But after getting the Rossi I have had a heap of fun and, for me at least, it seems to be a heck of a lot more useful and accurate (and safer) than the revolvers I once lusted after. It would be kinda nice to make up a Winchester rifle using your grand-dads bits and pieces.... and I bet he'd be delighted to know you were using them.... or were even just inspired by them.
Reflecting... wadcutters may have been better stabilised by your rifle, not so much because they compress the load / take up empty cartridge space, rather they are nicely shorter for their weight than a pointy bullet.
Sorry, when I mentioned different primers / dacron wads etc I did not realise you were using Trail Boss. TB ignites just fine no matter what primers you use, even in very cold weather, it ignites just fine -- even when you have a reduced load lying in the front end of the case because you are shooting downhill. If anything, avoid magnum primers as they are overkill on Trail Boss.
Ah... I'm glad you reckon primer brands aren't critical when using Trail Boss. I have quite a few CCI 400 primers to use up.