Recently I started playing the trombone again after a 20+ year absence. I used to play both trombone and baritone, using a 12C or 11 Bach mouthpiece. I had no mouthpiece for my current horn, a Conn 48H, so I went looking at the local music store. It appears that the standard default these days is the larger 6.5AL. I tried out both the 12C and the 6.5AL. Now given my lack of playing the last 20 years, initially I had trouble with both. But after a few minutes I was able to get some sounds going. While the 12C felt more familiar, the 6.5AL was more comfortable and had a richer sound, but it definitely felt large. I went with the 6.5AL. But now I'm wondering if I'd make faster progress, particularly in range development, with the 12C. Think I made the right choice? Were there others maybe I should have also considered like the Bach 11 or 7, or other makers?

Mouthpieces are a tricky thing. Some folks live and die by smaller pieces like the 11's and 12's. Some others, like myself, prefer much larger ones. 4's and 5's actually feel small. But then again, I'm a classical player.

When it comes down to it, you need to choose the mouthpiece that feels right for you, which it looks like you did. Don't worry about range development, that will come back over time. just work with the 6 1/2 as much as you can, and eventually a 12 will feel too small!

^^^he's right. Range will come. Truth be told range is all in air support!

I appreciate the advice. One issue I'm experiencing sometimes is air leaking out the edges of my lips, particularly on the right side. Any thoughts on how to correct this?

lol move your mouth piece a little to the right?
Really that will go away with time. Lip slur Lip slur Lip slur.

That's too simple, but it makes a lot of sense. I'll work on it. These days, I've got a moustache, so it's a little harder to see where the mouthpiece is sitting. Maybe I should look at shaving. Thanks for the advice.

tighten those corners. There's a video clip of Wynton at
http://www.bluenote.com/marsalis/player.html

Let the flash play, then click on the video. Let it lay. Notice Wynton's embouchure. Those corners stay locked even when he's in altissimo. (make sure you watch his whole solo)

whats the difference in outhpieces like the numbers craved into them because when i still rocked with a rental it had a 12C i believe and my new one i bought has a 6.5C (i forget if it has the c or not it might not) but the new one is harder to play high notes with. I don't know why or what the difference is but i'd like to know why.

The 6 1/2 is larger, which is why it is slightly harder to hit the higher notes. Don't worry, once you get used to the new piece, your range will come right back.

6.5al is way better than 12c. if you think it's too big try a 5g it's a little smaller but wont take away tone quallity

The 5G is larger than the 6.5. For Bach mouthpieces, as the numbers decrease, the mouthpiece gets larger.

following on what bob started to say, I have bach mouhpieces, a 7c, 12c, 6 1/2A and AL, and then a large bore megatone 5G. Those are listed from smallest to biggest in my inventory, but the 7c's cup, another words how far it sinks into the mouthpiece is probably, well, I would say about 3/4 of an inch at the deepest point. where as my megatone is close to an inch. Also the circumference of the 7c compared to the 5G is much smaller, if you put them side to side, which i just did, it takes a close eye to see that its not that big of a differece, but when playing it has a huge difference.

I tend to be of the opinion that the rim size of the mouthpiece should be based on your facial structure and comfort. The cup shape and depth and backbore have more effect on the sound. Remember, we're all different.
I use a mpc with a rim equivalent to somewhere between a Bach 3 and 4 rim, even for lead playing, because that is what is comfortable for me. but I use a very shallow cup and small backbore on the lead horn. I have no trouble getting a lead sound with it. I use a very deep cup with a very open back bore for orchestral type playing.
I'm not saying this is what will work for you. I'm just showing you an example of how there are exceptions to the "norm".

I'm the same way Steve. I play moslty (almost all) classical/orchestral now, and the piece I use is deep with a wide drill, and has a rather narrow rim. That's just the most comfortable for me. For jazz, my piece is a not as deep with a narrower drill, but still on the larger side. As for chamber, brass ensemble/quintet, and solo work, I use a piece with a slightly less deep bowl, slightly narrower drill, and slightly smaller diameter than the orchestral, but all larger than the jazz.

Mouthpieces simply come down to whatever you feel is the most comfortable, as well as what gives you the best sound. They literally will be different for everyone.

Example: Two of my students recently made the leap past the 12C (ughhhh finally) mouthpiece. One of them really liked the 6 1/2AL, and it showed. He sounded great on it, while the larger 4 and 5G I had him try were just too big and he did not like the feel of them. The next student tried the 6 1/2 and liked it, but then put in the 4, and right away his sound openned up, everything became clear, and I could see a huge smile as he was playing. So he was sold on it right away.

The only downside of being outside the norm like I am...
The only person I know of that will make a small shank mpc with such a huge rim is Doug Elliott.. And they sure aren't cheap! But they were worth every penny for me. No matter what I play, the rim is exactly the same. Makes going back and forth a breeze. After christmas, I won't be having to switch so much. I'm getting switched from the Showband to the Brass Quintet. woo hoo