I shoot a Bear Montana #50 @ 28" and I draw 26.5". Currently I'm shooting Gold Tip 1535 with 125 grain points. Total arrow weight is 375 grains which is about 8.2 GPP. FOC is 16.2 according to 3 Rivers spine calculator. I haven't hunted with this setup yet (or any trad bow for that matter) but I have some 125 gr Magnus Stinger 2 blades that I would use. I am confident that this would be enough arrow if I made the perfect ribcage shot on a whitetail. I am less than confident in how this arrow would perform on a marginal shot.
What I have in mind for a heavier arrow is Easton Gamegetter xx75 2117 shafts with a 250 grain VPA 3 blade. Total arrow weight with this arrow should come out to 645 grains with FOC of 20.2. GPP would be 14.2. Does this sound reasonable? My only concern would be that I don't want to be lobbing arrows at my max distance of 20 yards. 3 Rivers spine calculator says that I would be shooting 193 fps with the lighter arrow and 150 with the heavier arrow. Now, whether that's accurate or not I don't know. Anyway, I just wanted to pick your collective minds. I have limited experience in shooting arrows of different weights.
Why not retune your existing arrows with a 50 or 100gr brass insert, in place of the aluminum one.
Not only will you gain more weight, you will put it up front where it does the most good.
Would tend to believe that you current arrow is already far too stiff.
You may wish to consider utilizing a 100 gn insert for starters.
I'm no expert on this,but i've found 2117 to be pretty darn stiff. I'm having trouble getting them to work out of a 54@28 bow.
You could go to 3555 and probably get some good arrow flight,and up the weight a little.
As far as aluminum went 2016 would probably work.
Easton 500 FMJ, 75 grain brass insert 200 grain BH. That will take you over 500.
You don't mention how long your 1535s are now.
I'd take Friend's advice and load it up until it starts shooting weak. Then cut it back 1/4" at a time. I'm running 30" (full length) 1535s out of my mid-40 weight bows with 175 up front, drawn 28".
I just bought 1/2 doz Carbon Express 75s to get a bit weaker arrow and they still need 145gr on the standard insert to fly right at 30". I'd probably have to go all the way to .800 spine to shoot 125gr points.
Folks have given you good advice on how to increase the weight. Might be able to shorten your existing arrows or step up one spine, adding more point weight for greater FOC in either case. Might want to rethink your target weight, though.
A Montana isn't a high performance bow, and 14.2 gpp will likely give you a bit of a rainbow trajectory, even at 20 yards. Might shoot for something closer to 10-12 gpp. Still plenty of arrow for deer size critters, and will flatten the trajectory some.
All good points, I will add another. If you go with a brass insert and your arrow is long enough you can also add an external footing. If the footing is longer than the insert this will also effectively stiffen the spine much the same as shortening the arrow shaft. I am currently shooting a 31" arrow and use 2.5" of 2219 aluminum shaft for footing. Not only does this stiffen the arrow spine while retaining the additional shaft weight which allows me to increase point weight but the footing makes the arrow practically indestructible. Including my insert I am shooting 400 grains up front plus the weight of the footing. Total arrow weight is 765 grains, which is 15.5 gpp at my draw weight. From several Hill style bows this arrows shoots pretty flat out to my hunting limits.