I don't disagree on the added velocity hype. The company has thrown that carrot out there to lure the undecideds to purchase their product - my opinion. I didnt purchase it to get extra velocity, although I'm happy as a lark to have obtained it in my 300 Win Mag.
I'm no help as to what barrel manufacturers have to say about the product. I doubt many of the barrel manufacturers have ever heard about it, since most gunsmiths haven't heard of it either. But here's a thought to consider. If it lengthens barrel life, do you think they're really going to say they want people using it??? That would have been like GM building vehicles that wouldn't rust out in road-salt Michigan for 30 years back in the 1970s, instead of the typical 4-year rust through body panels.
My only concern, having used Gun Juice, is that one has to make sure the bore is clean-clean-clean, before continued applications because you'll just be coating the dirt/carbon/copper in the bore otherwise. For stainless barrels, it is recommended to clean - treat - shoot one round repeatedly 20 times. This is a tedious & laborious process. A process I believe the instructions don't really emphasize satisfactorily - possibly because it's a time consuming process and people don't want to hear that part of the bargain.
To the best of my knowldege and observation, Gun Juice lays down a very fine layer of resin during each shot and it takes 20 shots to throughly coat a stainless bore. I could tell the bore was cleaning up easier and easier as the multiple applications continued. A person wouldn't want to lay the resin down on top of a coppered bore, because then the internal dimensions of the bore will match the copper buildup rather than the manufactured dimensions.
The gunsmith in Canada that originally posted on the product on this Forum said he saw greatly increased throat life, and he's been treating his client's bores for years. He made no mention of any serious problems associated with the products use. I'm impressed with it based on my personal use to date.
So far I see no problems other than the time it takes to treat a bore. The good I see is my bores foul more slowly, clean up much quicker, I've gained 45fps MV in my 300 WM, and I believe that my bores are now more weather resistant than they would be without the Gun Juice treatment. Since my rifles see ocassional skiff duty on the ocean, I'll take any additional bore corrosion protection I can get.
For those that might be wondering - I absolutely do not sell Gun Juice or make money on it. I've purchased one 4 oz can for about $30.00 and have treated 4 different rifle barrels to date. It looks like there's enough product remaining to treat an additional 6 rifle bores, depending on the bore diameter. Small bores use much less product during the application process than the big-bores.