Erratic accuracy (more properly erratic ballistics as pressures and velocity spreads become excessive) is due to two causes. The primary cause is running the cartridge at too low pressures for the powder type to burn efficiently and consistently shot to shot; secondly reducing the charge fill-ratio too much, and its corollary increasing the amount of airspace in the case. Most cartridges perform at their best with a powder that provides full working pressures with a 90% plus fill-ratio, and it is generally regarded as undesirable to drop below 80% in larger rifle cartridges with 'normal' rifle powders especially with slow burning types.
The low charge / 'detonation' issue eventually became accepted after the many 'red herrings' of broken rifles caused by more common mistakes such as using the wrong powder or charge were weeded out and discounted. It's almost entirely confined to large case over bore capacity numbers such as the larger magnums from .300 Win Mag and upwards allied to an over-low charge of the very slow burning powders they employ. Look at most loading manuals' data for the Weatherby Magnums etc and you'll find the starting load involves a smaller % weight reduction over the maximum than applies to most other smaller cartridges with an injunction under each page of tables not to reduce the starting load any further. Even there, doing so would mostly just produce erratic loads not dangerous ones unless taken to extremes.
The normal practice in working up reduced loads with a conventional single-based rifle powder is to run them with a grade that has one, sometimes two steps faster faster burning rate than those normally employed, also to use powders which have a reputation for being easily ignited which in this bracket normally means single-base tubular types. These characteristics provide more consistent pressures. H4895 meets both criteria for larger cartridges such as .30-06 that would normally use powders whose burning rates are in the 4350 / Viht N150 or even 4831 / Viht N160 bracket. The fill-ratio drops, but the powder's burning characteristics overcome that and it'll burn happily at 40,000 psi or even less albeit sometimes very dirtily. Going back by several editions of the Speer reloading manual to the 1980s and earlier, the compilers inserted a single low-velocity jacketed bullet load for many popular cartridges. They nearly all used IMR-4198 powder in numbers like 308 Win, a powder whose burning rate normally sees it employed in the 222 Rem or low pressure large bore numbers like 45-70.
Ball powders should never be seriously downloaded as they are usually hard to ignite and need high pressures to operate efficiently. Very, very fast burning double-based powders such as pistol types operate very differently in the cartridge case from rifle powders and are designed to burn consistently despite often taking up a small amount of the available space. They can be used in rifle cartridges, but with lead bullets there being risks with jacketed types. In either event, doing research and knowing what you're about is a good idea.
Then, there are powders that have been expressly developed to provide low pressures and velocities safely in rifle cartridges with jacketed bullets. The old IMR SR4759 was developed by Dupont to a US DoD contract specification just for this purpose to simulate extreme range terminal ballistics in short-range testing with the 30-06 then the 7.62mm. The old pre Western Powders AA-5744 was likely developed for similar reasons and gained a reputation for excellent performance in large bore BPCR cartridges where it would simulate the (low) pressures and velocities of a case-full of black powder and also worked well in reduced loads in other cartridges being designed to burn consistently at a relatively low pressure. The IMR powder was withdrawn some years back and 5744 hasn't been available that I've seen since Accurate Arms changed ownership. However, 5744 was made by Explosia in the Czech republic and it is still listed as a European Lovex brand powder whose products are still imported ionto the UK in limited quantities. Its Lovex name is DO60, Here's what Explosia says about it:
RIFLE POWDERS
S040
High density, single base, tubular propellant suitable for .22 Hornet
and .30 Carbine cartridges and for other small shotguns. Also usable
in high capacity handgun cartridges.
D060
High density, double base, tubular propellant similar to Accurate
5744 designed primarily for .45-70 Government, .45-120 , and .50-90 Sharps cartridges and for reduced loads in all calibres rifles.
So, it may be possible to get hold of it. As mentioned, IMR Trail Boss is a specialised double-based 'fluffy' powder originally developed to produce low velocity low pressure loads in lead-bullet revolver cartridges that duplicated the performance of tiny charges of powders like Bullseye and Red Dot, but filled cases and made it impossible to use double or triple loads. This was for 'Cowboy Action' shooting where lead bullets and low MVs are mandatory. It has since turned out to be a great powder for reduced rifle cartridge loads especially with lead bullets but also some use with jacketed types and like 5744 for the large bore black-powder numbers. It's so bulky, it's sold in 9-ounce lots, that being all a standard Hodgdon / IMR 1lb bottle will hold. It was available in the UK, but I don't know if supplies have held up during the recent US shortages and problems in getting Hodgdon powders here. Vihtavuori also produced a Cowboy Action propellant N32C Tin Star that isn't anything like as bulky as Trail Boss and didn't catch on as well, but could also likely be used in some downloads. I'm not sure if it's still available.
For a downloaded 222 looking for Hornet MVs, Trail Boss is the obvious candidate. 7.5-8gn is a full case of this stuff and QuickLOAD predicts 8gn gives a 50gn Sierra SP 2,042 fps at 32,000 psi, but you'd look to start well down at 4 or 5gn charges and work up. 5gn takes up two-thirds of the case space and is predicted to give the 50gn 1600 fps at a mere 16,000 psi but still produce a 100% charge burn.
For 7X65R H4895 would likely work well using the previously described formulae. QuickLOAD says 43gn under a 140gn Sierra produces 2691 fps at around 43,000 psi. 33gn Lovex DO60 is predicted to produce 2628 fps at 46,000 psi and for really light work, 19gn Trail Boss (full case load) a mere 1790 fps. Both types are shown as giving 99-100% charge burns in a 24-inch barrel.
These are all QuickLOAD simulations, not field-tested loads, but give an indication of what might be possible - so treat them with extreme caution. It'd be interesting to see what people come up with. I'm sure there is a lot of experience around in downloaded 222 Rem loads as the little cartridge is simply made for this sort of experimentation in a safe manner if done sensibly.
