"these an lively, eager guns that reach ou so easily that they seem blessed with intelligence. I know that sounds too sweet to swallow, but the best of these carefully made
guns have a rightness of weight, balance and swing that approaches perfection."
Compare "(too) hard to swallow" which usually refers to something negative: hard to swallow 1. difficult to accept The terms of the agreement were hard to swallow, but I needed the work. My father's anger was very hard to swallow. 2. not easy to believe We found her excuse hard to swallow.
"Too sweet to swallow" would seem to be a (presumably American ) ad man's "slant" on "too good to be true", with a bit of mixing of metaphors... I reckon there is a play on the gun being "sweet" in there and they thought "Hey... too good to be true... too sweet... to swallow! Yeah, that'll work"
Et voilà ! A new phrase which doesn't really mean anything but which is easily understandable to the intended audience.
There are a few results for "too good to be true" here on WR which might shed some light.