I have a new to me vanguard which needs a 6-48 scope base hole cleaned up. I can't get a screw to properly thread without force. I have taper tap, bottom tap and plug tap. I appreciate any advice

Bottoming tap.

If the hole is a through hole, any of them will work. Blind hole takes a bottom tap as already said.

Use a plug tap from which the point has been ground off, that will get you to within 2-3 threads from the bottom of the hole then you can complete tapping with the bottom tap. You need the lead of the plug tap to reduce the pressure it takes to turn the tap thus less danger of breaking the tap. The bottom tap has the least lead of the three and is meant to complete the threads from the lead of the plug tap. In nearly 40yrs. as a Journeyman Tool & Die Maker I did all my tapping with plug and bottom taps, never used a taper tap. In most instances with a taper tap especially tapping blind scope mount holes there is too much lead to allow getting a proper start and you run the risk of stripping the first thread. Anytime you have to force anything that is telling you to STOP there is something wrong, continuing will break the tap, strip the thread or damage the screw. Taps are inexpensive enough versus the cost of removing a broken tap that I nearly always started with a new tap. You can feel the difference in the amount of pressure required with a new tap vs. a used tap. Brownell's sells a tap they call their "premium" tap, the tap is manufactured by Reiff & Nestor a company that has been in business since 1908. R&N has a reputation for producing the best tap in the industry, this due to the fact that they control the tooth to tooth spacing on their taps to a closer tolerance than any other manufacturer in the industry. Try one and you will see the dramatic difference in reduction of effort required to tap a hole. I won't tell you I don't remember ever breaking an R&N tap, but their were very few instances. Some of the parts I made required tapping 80-100 #0-80 holes in stainless steel, I didn't even consider using anything other than an R&N tap.

NO way I could have said it all better myself... Kudos re: that post...

I learned a thing or two in 40 years, prolly D&T thousands of holes over that time period. Not only doing my own work in the Toolroom but designing and building machines for production quantity tapping on the manufacturing floor.

this again

Thank you gentlemen for the advice. All taps are brand new, so I hope all works out......

Would this be a good job for a thread chaser? If you could find one in that size...

A thread chaser is normally used for the screw, unless you are talking about a tap for that

Just curious, but since Howa barrel threads are metric, are the scope mounting holes threaded for the metric equivalent of 6-48?

I've made thread chasers out of matching bolts to clean up a threaded hole and they always worked great. However, never made one as small as a 6-48 so not sure how well it would work out. I've always heard using a thread chaser was safer to the threaded hole as it won't screw up the original threads like a tap possibly can.

Bob