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I too bought a Model 1 Sales 300/221 kit w/ assembled upper, and have had the same experiences.
I've read about the opened-up port in the standard shorty location, but that seems like a marginal fix to me. It will probably work with subsonic, and to get best success, use a chrome-plated AR-style (lighter-weight) bolt carrier, a lighter weight buffer spring, and don't plan on shortening the barrel any...
I want my lower to work with other uppers, without changing out the buffer spring, so that's out. I want to be able to utilize a heavier bolt carrier and shorter barrel, so now we're back to marginal operations...
In my opinion, if you're going through all the trouble to modify it, make it as close to the SSK upper as possible: TIG weld the gas port under the front sight, drill a new port at the pistol position (7-8" depending on whose gas tube you use), and for best operation, get an adjustable gas block.
This is more involved than just drilling out the port, but it will be the most flexible in terms of working with the widest selection of loads, and with the least amount of fiddling with the lower and bolt. Theoretically, these modifications should keep the 300 upper modular, as the whole AR system is meant to be, meaning you can swap out bolt/bolt carriers, and put the upper on different lowers with a high probability of it working...
Lastly, there have been some posts here in this thread which show the downfall of the JP gas block. (the allen wrench) To be fair, the JP block was designed for use on target guns at the match ranges, where mis-laying an allen wrench or leaving it at home will not get you killed. But, if you have a 300 for field use, I would think you'd want an adjustment device that is always attached. Again, I look back to the SSK upper, and note that they install a large knob with a coin slot that shows up through one of the holes in the upper handguard. I will be working on something that will work similarly with the JP gas block on mine...
My next problem to overcome is how to work the adjustment device around a solid 4-rail handguard without cutting holes through the Picatinny rails... When I get it all sorted out, I'll post some pictures here.
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