I'll jump in since I shot production last year and am doing it again this year. I think that makes me the longest running production shooter in PRS history
so here is my take.
Last year I shot a RPR in 6.5 with literally nothing changed on it besides an APA little bastard brake and using MDT metal mags. The brake helps over the stock ruger brake for recoil and the MDT metal mags are less prone to occasionally popping out if you are pressing them against a barricade. The MDT mags let you handload beyond 2.80 COAL which can help accuracy in some cases, but given that some of these $1999 production guns don't come with a brake, mag, or even scope rings I thought it was well within the spirit of the rules to change that stuff out.
At the PRS finale I realized that I was at a noticeable disadvantage to a nicer production gun like the MPA with that RPR. Not because it won't shoot well, but because its got a round foreend that rolls on a bag, it can't clip into the fairly ubiquitous ARCA tripod stages, the balance point of the gun is way too far back, and ready to fire it only weighed 13.5 lbs. It didn't stop me from winning, but clearly it could be made better with a couple simple and cheap improvements.
This spring I got 6mm RPR when they were $649 on gunbroker and bolted a flat sawtooth arca rail under it, added some keymod weights to the side of foreend, and put an even bigger (heavier) big bastard brake on it. It hasn't improved accuracy when shooting groups, but made a -huge- difference in positional shooting since it balances on a bag and you don't have to give it a ton of shoulder just to keep it from falling backwards. If you have a MPA or one of the other nice production guns you get a 26 inch M24 profile barrel that does the same thing for you already, so I think I'm living within the spirit of the rules by bolting on some weight to make it balance.
During this year I've managed to crack the bolt shroud on the RPR, so I replaced it with an aftermarket one and marred the pistol grip up so badly I decided to change that out too.
All considered I'm still into this gun for < $1000 and anyone could build the same thing off the Cabellas rack this weekend, which at least to me is the intent of production. I've never had any one else ask about things like the rail or weights being upset, but I've let at least a dozen guys with bone stock RPRs shoot my gun versus theirs off a barricade so they can see how big a difference that balance point makes and then they probably go out and buy them.
Things can get a little fuzzier though and you just gotta use your best judgement...
You can see in pictures of Doug (Ruger sponsored shooter with a RPR in production class) that he changed out the round forearm on his 6 creed RPR to the one you get on the magnum guns. If someone wanted to be a dick that might technically be against the rules because its the "chassis" and you can't actually buy a RPR that way. I personally don't care, its no different than bolting on the sawtooth arca rail you can buy.
A few folks have suggested he's not shooting the normal 24 inch and profile 6 creed barrel that anyone else can buy off the shelf, but you never know if its just sour grapes because he's beating them with a RPR.
I can't say I've set my gun next to his to compare but production class is self-policed and the rule on the barrel is pretty clear. I doubt a Ruger sponsored shooter with his background in pistols would try to cheat on something like production class rules, that would backfire pretty hard for them.
So run what you brung, change the grip out, handguard, whatever, it doesn't bother me - just keep it within the spirit of the rules so anyone else could run the same thing and not spend open class money to do it. Sure as shit, the grip you run on your rig isn't going to be the difference between me losing or beating you in a match, so hope to see you at the next one!
