There are very few moderns weapons at either the Days Inn or Agricultural Hall shows. In fact, a good portion of it is militaria and not guns at all - the guns that are there are mostly antique or C&R guns. The Days Inn show is very small - maybe 200 tables. The Agricultural Hall show is much larger, probably 800-1000. Be advised that they don't allow children (I think under 10) in at all, so don't bring them if you plan on attending - would suck to drive all that distance and not be able to go in...
I haven't made it to the at the Expo Center yet. I believe it is a fairly new show. I heard bad reports about the early ones - the promotor advertises 1500 tables, but that is the amount of tables available and I heard a good portion were empty for the first couple. It may have gotten better, however.
The show at the NG Armory is OK, the one at the Valley Forge Convention Center has a similar sort of makeup, but is at least 2x as large - next one is Sept 20-21.
However, if you really want to go to a big PA show, you need to hit the semi-annual one in Harrisburg at the Farm Show Complex. The next one isn't until December, however. Be aware that there are two different shows at that location - the "big" on is put on by MAAC, the smaller one by David Promotions.
Rocko