I have only shot 2 577's in my lifetime..when I was helping the local Westley Richards shop do some testing for consignment guns..if I remember correctly they were in excess of 11 lbs--around 11.5 to 12.5--know my WR in 470 Nitro is around 10.5 lbs. and it is very pleasant to shoot..think you would be very close at 12..but am sure others with more experience can shed some more light on it..

Also I would check Guns International..look at what the .577's are weighing in at..

Ripp