Weatherby SA 459 Question

I just purchased a new, Weatherby SA 459 tactical 20 gauge. I guess this is the newer version of their SA 08. Nice gun. Sharp looking, light, pistol grip and 18.5 inch barrel. Black, synthetic stock.

The gun comes with two pistons. One is engraved "light load" and the other "heavy load." Now, there is nothing in the owner's manual or anything I can find online that describes what differentiates a light from a heavy load. So, I took it out to fire this morning with the heavy piston in and some Winchester Super X, low brass, 2 1/2 dram powder field shells. Didn't even cycle the bolt. Took it back and put on the light load piston. Same shells, only partially opened the bolt, leaving the spent shell in the chamber. Switched to Estate, high brass #4 shot with 2 3/4 drams of powder and was able to rapid fire 5 rounds.

OK, it doesn't like low brass loads but my question is this: Does anyone know what the parameters are or what would differentiate "light" from "heavy" loads for this gun? If it takes at least 2 3/4 drams of powder to cycle the bolt with the light piston, when would I need to install the heavy piston? Oh, I forgot to mention the shells were 2 3/4" shells I had on hand.

Shot my buddy's SA=459 yesterday. He says that when he tried the gun out, same thing happened to him with all loads except slugs, that the gun wouldn't cycle with the heavy piston. He changed to the light piston to handle everything, and the gun works well (but don't use that Winchester junk, it doestnt work well with anything even pump guns.) Unless you are using slugs exclusively, stick with the light piston.

Thanks for the info. I have a friend of mine who also purchased one and we've kind of come to the same conclusion.