I asked the same question in this forum back in January and got a range of answers that helped me establish some parameters for my own GPR 54. As I recall they ranged from a low of 70 grains of FFFg to a high of 120 grains of FFg- all with PRB. Got my GPR too late for our deer season so it's all theoretical, but I've been playing around in that range, along with 100 shots or so with the 435 grain TC Maxi-Hunter (the short one in recognition of the slow twist in the GPR).
I have been shooting 30 grains of FFFg with a .535 PRB and Oxyoke prelubed .010 patch for snowshoe hares for about six weeks now. Very accurate and effective, but I had to crank the rear sight all the way up to hit point of aim at 30 yards. Fired patches look fine with this load, but tend to start tearing up if you get past 50 grains. This patch is definitely too thin with a .530 RB.
I've been shooting 70 grains of FFFg with the .530 PRB with an Oxyoke prelubed .015 patch for general target work, and the combo seems especially practical. I fire as many as 30 shots without swabbing the bore, and the last ball seats as easily as the first. I wouldn't hesitate to use this for deer at 50 yards based upon penetration, etc., but drop is noticeable at 100 yards- my probable max range for this rifle. Increasing the charge to 100 grains of FFFg improves trajectory but doesn't seem to worsen fouling, so 90 or 100 grains of FFFg with this ball and patch may be my hunting load, even as I continue practicing most of the time with 70 grains to save a few cents. Fired patches look fine throughout. Bore fouling is much worse and swabbing is required every five shots or so with FFg.
I recently picked up a pack of the Oxyoke prelubed .018 pillow ticking patches. It's a real tight fit with the .530 RB- takes a couple of whacks with the short starter even to start the ball, then noticeable effort to seat all the way down to the powder. Effort goes up even more after 5 or 6 shots, and swabbing is needed. Switch to FFg, and I have to swab every other shot unless I want to make ball seating an Olympic event. It's an accurate combo to shoot, but doesn't seem to offer any advantages over the .015 patch, even as it is more troublesome to load.
The 435 grain conical is interesting, but I'm not sure it's needed for deer. It shoots 3-inch or so groups at 50 yards with 70 grains of FFFg, but hits about a foot low compared to the same sighting with a PRB over 70-100 grains of FFFg. Increasing the charge to 100 grains of FFFg raises groups noticeably, and flattens trajectory to 100 yards a fair bit. Groups at 100 yards with the 100 grain load hover around 4 inches, while they are more like 8 inches with the 70 grain load. I am not convinced that a conical offers any advantages over PRB in terminal performance on deer, and a question to that effect in the General Interest set of threads hasn't yieled any answers to date.
I will say that if you are even a little bit recoil shy, it won't matter if there is an advantage in terminal performance with the Maxi-Hunter because you simply aren't going to shoot it well enough for 100 yard shots at deer. It's a real "attention getter" on the back end with heavy charges in the comparatively light GPR 54- reminds me of 3" slug loads in a light 12 gauge pump. Call it fun to shoot, and you're revealing your attitudes about whips and chains in the bedroom.
