Bawapp
Oh Geez! you almost said a nasty word - Bore Butter - Bore Butter is water soluable and can and will contaminate powder if it gets in contact for a long enough period and if there is enough BB.
I am going to assume that you are only using a very small amount to lubricate the post.. That in itself should not cause powder contam.
If you might be using it to lube or swab the barrel - that could be a problem especially if left over a period of time... How are you using it?
PowerBelts - they, the bullet part, do not normally contact the barrel until the gun is firedwith pressure cause the bullet to grab the lands.
What kind of gun is this?
Never mind just saw that it was a CVA Optima... depending on how you are using the BB this also might be your grouping problem...
When I pop primers there are sparks visible at the barrel end.
This really should set your powder off... If you want to go back to square one - Clean the barrel down to bare metal - dry it make sure it is dry - no lubricants no nothing - pop a could of primers - load the powder - load a clean dry PB and shoot it - if it goes off load a second now swabbing nothing and shoot it. If they both go off then I think we are on the right track...
another thought as remember your posts - a second primer or third will ignite the powder - each of these primers will cause some drying of contaminated powder and eventually it is going off - that is what I am thinking... so I am still leaning to the contaminated powder thought...
don't get me wrong about BB either it is a good product for which it was designed - I use it in my guns to reduce fouling but, I probably apply it a lot different than the normal duck...