Jim, QuickLOAD doesn't allow for a barrel-cylinder gap entry, but you can vary the barrel length. For semi-autos I enter the exact barrel length, and the program deducts the OAL to compute a corrected barrel length. For revolvers I usually enter a longer barrel length that allows the corrected barrel length to equal the actual barrel length. It sounds complicated (at least the way I describe it), but it's actually pretty easy.
For this simulation, I used 7.5gr Power Pistol (giving a computed psi of about 19,400), and a 125gr Remington SJHP bullet seated to give an OAL = 1.420".
Corrected Barrel Length --- Computed fps --- % Powder Burned
2" --- 920 fps --- 72.9%
3" --- 1058 fps --- 80.0%
4" --- 1153 fps --- 84.2%
5" --- 1224 fps --- 87.0%
6" --- 1280 fps --- 89.1%
It would be interesting to run a test on the same revolver, cutting off 1 inch of barrel length at a time to see if these numbers hold up. At first glance, they seem to me to indicate more velocity due to barrel length than I have observed in real life, but maybe I just wasn't paying attention!
With QuickLOAD you can vary almost any input variable such as seating depth, case capacity, bullet length, bullet weight, powder used (if it's in the database... most are), barrel length, bullet base type, moly-coating, etc. There's even a "weighting factor" you can use to adjust the computations to match the actual results from your weapon. I've found that the more precise I am with the data I input (especially things like case capacity and bullet seating depth), the more closely the computed outputs match the "real world"
results measured with my chronograph.
Of course, there are lots of warnings with QuickLOAD that it can compute bad data (essentially: garbage in = garbage out), so you have to be careful with it, and should compare it to other published sources. Also, I usually start at least 10% low on any new load and work up slowly. And if I hit the velocity I was looking for at a lower-than-expected powder charge weight, I stop right there, always assuming that there's no free lunch.