If you are reffering to speed, there is no calliber that can beet the .177. You can get a .177 gun that shoots anywhere from 150 FPS to 1200 FPS. There is a catch though, because energy is calculated by mass X exelleration, you can learn that where the .177 has high speed, it has very low mass. If you got a .20, .22, or .25 calliber gun, than you would have very low speed but very high mass.
Final Match Pistol Pellets by H&N
Which model?
.175 often refereed to as .177
Most, if not all Red Ryder BB guns use .177 size BBs.
Air guns and air rifles come in many sizes. BB guns are .177. Pellet guns & rifles are made in .177, .20, .22, .25, 9MM, 357 Cal, 45 Cal and 50 Caliber. The last four sizes are usually restricted to the PCP (PreCharged Pneumatic) class of air guns. (Big Boar)
Your question is not clear. What are you converting from. Air guns are already .177 unless you have a larger barrel on them. In that case you just change the barrel to .177. If you are converting for "AIRSOFT" then it can not be done successfully.
There are over 100 different model .177 BB guns. It would help if you stated the model you are asking about.
It usually says right on the barrel. Single shot / single load pistols are pellet pistols. Most BB guns have a reservoir for the BB's. There are a few that shoot either a BB or a pellet. You will have to look at the instructions or the display box to see what it says. Pellet pistols are more accurate than BB pistols. Most BB pistols will say .177 BB. Pellet pistols will say .177 Cal and not say BB.
Lets get something straight. There are BB guns / Rifles and there are Pellet guns / Rifles and there are a few BB & pellet rifle combinations. Most BB guns are .177 Caliber, there are a very few that are .22 caliber. Pellet gun & Rifles come in .177, .20, .22, .25, 9 mm, 40 Caliber, and 50 Caliber. The last 3 calibers are usually high powered air rifles in the PCP class of rifles (PreCharged Pneumatic) they require a special pump or SCUBA tank to charge them. So Yes there are calibers much larger than .177
Pellet guns tend to come in two standard caliber sizes. The first smaller caliber is the most common for pellets guns and is .177 caliber. The second is .22 caliber that can be found on some higher powered pellet gun models.
You need to consult your owner's manual. Some BB guns can shoot pellets, but some cannot.