cayuga said:
I'd hate to even guess as to what the max load for the rifle is. I too have been interested in a .40 caliber GMB for target and varment shooting. I will be interested to hear how it shoots for you... Perhaps call Green Mountain and ask them what is the max load that barrel will handle. Good luck with the rifle. I will be waiting to hear the results...
The book that came with my 32" GM .40cal barrel, which I have installed on an Investarms Hawkens stock, is the same one that came with every other GM barrel I have gotten. That book just carries the the same warning about not loading over 120 grns of Ffg, which I'm sure is just for liability reasons and is by no means a suggested maximun load for the 40cal.
You would like the .40cal, cayugad. My GM .40cal is a tackdriver, but so are all my GM barrels. However, it might be the most accurate barrel I have. With no wind, it will easily shoot quarter sized groups, or less, at 50 yds, from the bench. I use a .395 ball, .016 pillow ticking patch lubed with Stumpy's moosemilk and 40 grns of KIK Fffg. It is a heavy rifle but because of this, it just doesn't have hardly any recoil. For squirrel hunting, I used 30 grns of Fffg and you could literally shoot them in the eye from within the 30-40 yds from which most of my shots occured. I have shot up to 75 grns of KIK Fffg, with the groups only opening up slightly, still easily within a 3-4" circle. Again, this is with no wind because a breeze will really move the little ball.
From my experience, I would say one could use the 75 grns of Fffg blackpowder for deer hunting and have great accuracy. I'm not sure how much velocity, or energy, this would produce but you should be able to put the ball where you want it, within 50 yds, and I wouldn't shoot beyond that. A .40cal ball is pretty small. Here in Illinois it's not even legal for deer so I won't have the opportunity to try it.