As Okie mentions above ...... the Kolpin mounting system is poorly designed, and will eventually crack the plastic boot. Besides that, the Kolpin 'Gun Boot' (one size fits all) is a poorly designed product also. They need to be modified to even get mediocre use satisfaction.
Make your own bracket, and stabilize the barrel part of the Gun Boot with an additional bracket.
I'll post some photo's later.
With my custom bracket being about 12 inchs long and about 8 inches high and a width that is a snug fit for the Kolpin case to push down into and
I did not need a barrel brace to keep the case from vibrating because the bracket itself does not vibrate. The Kolpin one size fits all universal fit brackets themselves also vibrate.
I used the Rustoleum black truck bed liner instead of paint on the custom bracket. The Rustoleum truck bed liner comes in different colors and I've found it's very good instead of paint for such and even on the ATV undercarriage shields, carrying racks, winch brackets and other places on metal around the farm and ranch.
The Rustoleum brush on type bed liner that I use is from a gallon can, stores real good for long term, year or more, even after being opened if can is re-sealed good, dries fast at 70 degrees F for a second coat. But be careful, it has a Acetone base and will melt paint that has not been fully cured, so make a test first in a small place if applying over paint or primer because it may soften or lift paint and might do the same to plastic. I use the disposable cheap paint brush and good latex gloves.
Read the instructions on the container or you might can review such online.
It don't show scratches and don't chip like paint and has a kinda rubber texture, bonds really good with minimum prep to a rusty metal surface.
I have not tried the spray on types, suspect the left over would not store very long in the aerosol cans.