Extended probe and flow-through bolts apparently work equally well. Each sets a pellet a tad deeper than a stock bolt. Some prefer the flow-through becuase it sets the pellet by contacting the skirt. This can be a good thing since the hollow cavity in a pellet may vary from manufacturer to manufacturer causing pellet seating to be slightly different when using an extended probe bolt. I use an extended probe on one of my 1377's. It works fine.
"First: What is gained by installing an extended probe?"Nothing unless it's done properly. Even at best a 10-20fps increase is @ all you can expect with any degree of confidence.The entire concept of the extended probe is to reduce flow restrictions and seat the pellet to the proper depth. That means that you MUST know what pellet your gun prefers because the depth of the rear recess varies widely from pellet to pellet even in the same manufacturers product line. Then the length of the probe can be adjusted to seat the pellet immediately ahead of the front edge of the transfer port where the greatest efficiency will be obtained." Which one would you recommend?"For the reasons stated above one that is custom made for the application. Since I appear to have been the first to have written a how-to on the long extinct Airgun Letter Forum I can tell you how to do it properly----if you have access to a lathe. If you want to extend the probe on a .177 cal. then you'll need 1/16" spring steel 'music wire' from a hobby shop. For a .22 cal. 3/32" is preferred. Chuck up your original bolt in the lathe and drill it out with the proper size bit. Be sure to drill at least 1/4" past the o-ring groove because the hole weakens it and you will need to seat the new probe past the groove to regain the original rigidity & strength. I then cut a length of the music wire that I'm certain will be long enough for me to grind it to the finished length and---leaving the bolt in the chuck---glue it in with either JB Weld or one of the permanent LocTite products. I press it in with the tailstock ram so hydraulic pressure cannot force it out and leave it overnight to cure. A couple of days if time isn't pressing. When finished re-install the bolt to the barrel/breech using a new o-ring on the bolt. Insert the preferred pellet and seat it to see how much of the new probe must be ground off to attain proper seating as described in the 1st paragraph. There are other things that can be done to refine the face of the bolt to smooth the flow but I'd be here for hours typing coverage for all minor points so just think about it and use your imagination a bit.Don't have a lathe? This is a good excuse to get one in that case!;o) Tom