30" barrels on skeet guns, in my area, are very common these days....its hard to believe it has anything to do with the barrels ( unless you're used to shooting a 28" gun ...and this is the only 30" gun you have).
It might be the choke ...or it might be the shells.../ but you said point of impact is fine...so the only other variable is the choke or the shells.
The pattern board is the only way you're going to be able to see if one choke or one brand of shell is giving you holes in your pattern. I'd also pattern both skeet chokes ....and if you have 3 or 4 skeet chokes, then pattern them all...
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The other thing ...is the dynamics of the gun...20ga guns are often built on lighter receivers than their 12ga cousins....sometimes meaning they weigh almost a full lb less than their 12ga cousins. Consistency ...in your swing and follow thru ...can really be affected by weight...
Case in point - I have 4 of the Browning Citori XS Skeet models ( 12ga, 20ga, 28ga and .410 )..../ they are all 30" barrels.....12 ga weighs about 8.5 lbs....each of the other 3 guns, built on the 20ga receiver weigh about 7.5 lbs....so what I've done is put 8oz of lead tape ( lead golf club tape) inside forend ...and 8 oz inside the stock ...making sure I did not change the balance point....so all 4 of my guns feel the same...
Sometimes its the little things too ....maybe the throat area on your 20ga just feels a little different than your 12ga version....( or maybe its psychological ...that you know you're shooting 7/8 oz loads...and its getting in your way..../ .....and its funny that you're having an issue with your 20ga....because I tend to have issues with my 20ga as well...but I don't have the same issues with my 28ga...( and I shoot higher scores with my 28ga than I often do with the 20ga...and I know its in my head / but I've had a hard time getting over it )... I've shot way fewer 25's with my 20ga ...than my 12ga or 28ga, and other than mental, I don't know why .....( but I know its not my chokes or my shells .....( now the .410 tortures me too ...but that's a different issue on effective patterns)...and the .410 is really a mental barrier for me .
I know you've been shooting skeet long enough to know that score consistency comes and goes...especially as we get older../ and there are some days where it just does not work ....and its usually because one of my fundamentals has changed ( not rotating lower body as well, lifting my rear foot heel, rolling my shoulders, something....)....and usually it takes someone behind me, to tell me what is going on....and settle me down.
But first things first....veryify your patterns with both the chokes and the shells...and pattern the IC chokes as well...with the same shells. If everything looks good...then start looking at guns specs ...and your fundamentals...and your mental game.
