In my experience in respect to the 1077- about 650fps is all you can expect to extract from it without starting to make custom parts. Even at 650fps (using 7.9gr pellets) alot of CoČ gets wasted- maybe 24 shots from a 12g caplet or 150-ish full power shots from an AirSource 88g cartridge (200-ish shots with the last 50 being 10-20% lower in power).
Also, to note it, Crosman sells the 1077 with a "maximum" velocity rating of 625fps using (we must assume) 5.5gr Hobby-style pellets- and some guns don't even get to that 625 rating. So, 650fps with 7.9grains is nothing to sneeze at necessarily- but it's not arrived at with only one step- the valve ports must be expanded, o-ring in valve detent checked (often it is missing from the factory), tighter spring in valve detent, a stronger hammer spring or weight added to hammer and perhaps a bit of valve spring modification could help a bit- but then the valve starts to get really unbalanced and wasteful of gas.
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