Rick,
I have accumulated a fair piece of experience with the .20 cal...
First, this is NOT a longrange caliber... IMOP > 600 YDS...Some do... But ya got to be honest with yourself about distance.
I actually keep'em under 450 or so... No matter what the wind is doin. The .20's are at their best < 500yds...And cases like you are interested in just arn't needed... Some will disagree.
Your courious about accuracy in two truly overbore cartridges as far as based on a .20 is concerned. Obviously the 20PPC is less overbore than the 20BR.... With that said they 'can' shoot well with GOOD bullets and GOOD equiptment/shooting technique.
But I KNOW the 20BR is just not a good platform...,Really getting feathers ruffled now...!

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Cases like your interested in will NOT be more accurate than a well tuned .222/223 based 20 cal. Even the 20ppc will burn WAY more powder than is nessisary / muzzle blast and recoil / IS much tougher on the barrels throat and will flat out destroy a GOOD barrels throat < 1000 rounds.... I HAVE SEEN IT..!
A 20BR is WAY WORSE...!>>> Have seen a Pacnor .20 3-groove SSS barrel throat BLASTED 1 1/4 inch of the throat and the throat had no rifling for 1/4' ahead of where the throat 'was'. This from a 20BR...Even the 204Rug is a bit on the overbore side of things... Some are bound to disagree...
Think it's just a Pacnor issue.... Nope. A Shilen 20BR did not fair any better, <600 rounds showed SIGNIFICANT throat erosion.
Look at the .222/223 based 20 cal cartridges.
Lottsa choises here... I really like my .20-222... Accuracy.. YES. Good barrel life YES. Velocity....Yes..! Not even hot loads and I get honest 4000 with a 32gr pill. 3750 - 3850 from a 39/40gr pill... Easily.. No bull! Heck, I get 4300fps outta a factory Savage action/Pacnor 3-groove .20-222 with the 32gr V-max. Bolt just gets 'clicky' accuracy falls of a tad to just .5 MOA but boy ZIP... Crows go POOF..!
Question is, are you 'stuck' with a .308 bolt face..??? Don't have a .223 bolt face rifle..?
Could someone please explain the differance between 20BR and 20PPC? Which one is more accurate? which one is easiest to load for? thanks.
Rather than explaining the differences... I'd recommend stear'n clear of these.
Probably the most user friendly easy to reload for IMOP is the 20Practical.... Based on the .223 case... Oh AND it SHOOTS.!
Hope this helps ya getting started in the .20 cals in a more well $$$ spent manner / less frustration...
cale