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Nobody I shoot with has a shotgun comp.
If you do go for a shotgun comp, remember it moved you into the open class

I use Federal LE Tactical (full power) in my benelli m1 and I find them to be quite accurate. I buy them online and have them shippedto the house.
As far as finding slugs in town, most places seem to carry the same brands and not much selection.
I would check out ammo-to-go online (they are in Brenham) and see what they have. Their prices are OK

This may not be correct, but aren't those Brenneke slugs for rifled slug barrels?

http://www.ammunitiontogo.com/

If you are coming to the WHIDPA multigun match, 71/2 (lead only) should knock down the steel at the distances we shoot at. That being said we allow nothing larger than #4 (lead)(i.e. no buck) for safety reasons.
We occasionally have a texas star at 15+- yds and those with open chokes, like a unchoked 870, sometimes have trouble getting sufficient pellets (7 1/2 lead shot) on the moving plate to knock it off. In that case some #4 would help.
3 gun matches really need you to pattern your shotgun with a variety of loads and chokes, so you will make wise choices.

The longest distances will be the slug shots. AT WHIDPA we reguarly do 50 yd slug shots and are looking into ways to do 80 yd slug shots.

There are other 3 gun matches that allow larger than #4 shot. Check rules for the match you are going to.

Bob