Watch papercartridges on YouTube. Brett Gibbons, who makes swaged hexagonal bullets, is a poster here. He makes the very special cartridges too. His excellent movies will answer many of your questions.
The general consensus is that nothing comes near the Parker-Hale version. Opinions and experiences with the Pedersoli versions differ widely.
I've had two Parker-Hales since 1986, the older is from 1980, and the workmanship is exemplary in every respect - particularly the barrel blacking which is almost matchless on a mass-produced firearm. Remember that this rifle is forty-six years old -
Friend Dungspreader, formerly of this site and late of British Columbia, had hair-raising and expensive experiences with his Pedersoli, and TBH the only one I've actually seen for real was a real run-away-now job, especially given the high cost here in UK. The Euroarms version, made for the most part with P-H barrels up about 14,000 seem reasonable enough. One I see at our range has beautiful wood but the owner struggles to hit the backstop. It worked fine for me, however. As ever, the 'nut behind the butt' needed some adjustment, that's all.
David Minshall here, from the Muzzleloading Association of Great Britain, and friends Fleener and dave951 are THE resident experts on this finicky shooter, but in general the Lyman 535gr multi-groove cylindro-conoidal bullet will shoot as well as the extremely costy-to-buy hexagonal items. This style of bullet acts like a compression bullet and bumps up into the hexagonal shape we know and love - mostly. Paper-patching works, too. More an art than pure science, that too may lead to much hair-tearing, but less cleaning, although the general advice is to wipe the bore AFTER loading, but before shooting to offer the bullet the cleanest route out. As ever, opinions on that differ from shooter to shooter. The best results are obtained via long experimentation with your particular rifle - which WILL be different to mine, and his, and his and his.
Mine likes 70 - 80gr of Swiss #4 - 1.5Fg - for plinking. You can happily shoot 900 yards with a 90gr load and the issue sights, but whether or not you can hit anything out there is moot. Most people fit spiffy tang sights, like those from Soule or Montana Vintage Arms - figure on $600 -700 on top of the basic cost for them.
Try and find a REAL Parker-Hale version with a serial number less than 10,000. They will generally be well-looked after and little shot, mainly for the reasons you've noted. P-H never actually MADE 10,000 - that is the extent of the full range of P-H repros and the Whitworths will all be in there somewhere - my current rifle is 420 from 1980.
After 10,000 or so, and the sale of all the P-H assets to Euroarms in Iddly, things got complicated, as all the P-H-made parts ran out. The genuine P-H barrels of all kinds ran out at around 14,000, but may be found marked Parker-Hale Birmingham, but with Italian proof stamps and breech plugs. Confusingly, even a few of Pedersoli's early rifles are like that.
And after that they are totally Italian-made, lock, stock and barrel. In my experience the Euroarms versions - all-Italian-made - are very nicely put together and can work well. I've never seen a Pedersoli WW in action, but from reading comments here it seems that the QC can vary alarmingly on a >$2000 rifle. The example that I saw here in UK brought a whole new world of meaning to the description 'plain as a pikestaff', and I was substantially less than impressed. In all my seventy-one years of shooting, I've never seen a less-figured piece of wood on a rifle at any price, let alone that demanded by the dealer for this example. It really did look like the wood on my Red Ryder of blesséd memory, but with less figure. So if you are buying, then you'd really HAVE to handle ANY rifle you intend to own.
And please read the archive back-numbers here - they ARE extensive, to say the least.
I'll no doubt be reminded that the replications to NOT copy the Whitworth rifles that went to the CSA during the Civil War. The real things were only two banders, similar in length to the P58 rifle. None were ever smoothbore, so they were never described as muskets.
And if you get a minute, take a look at this post from the archive - it's me and my latest and oldest P-W rifle with a few useful posts and lots of comments from stage left -
Parker-Hale Whitworth - 1st shot and lookie-round...
Here's a 200m group shot a while back with 80gr of 2Fg, and as dave951 notes, the bullet holes are clearly hexagonal - this from what started off as a round bullet.