Jim it sounds like to me you need either John or I to shoot it in a match for you so we can tell you just how good a shot you are.
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The Ultra Mags in the 500 count mixed lot batch tins aren't as accurate at 100 yards as the 1250 count Brown Boxed versions which are made from the same die though. They are OK for informal plinking at up to 50 yards. Past that there comes the well known Crosman ammo fliers.
One of my unregulated Gen1 Marauders and my new Field and Target edition shoots lights out SUB half inch at 50 yards with them and my Gen1 unregulated FX Royale shooting at 1000fps (YUP) does 1/4-5/16" at 50 yards with those. My 177 CareerII 707 can get moa at 100 yards at 1055fps (YEP) with them too. Hadn't shot my 177 Sumatra at 100 enough to give an accurate assessment but it keyholes at 50 as long as it stays below 1055fps.
Many other 177s can't do too well at 100 yards unless they got the power.
Man that sounds cool! Would love to see pics of your 100 yard range and those .177 groups.