A few notes about machine screws and where to buy them:
Don't buy ANYTHING that has a "slot-head".
Put a decent chamfer on the clearance bolt-holes. Some screws, in
fact most screws, have a root radius, which is why you place a washer
under the head.
Try a proper supplier, like Fastenal or somebody's "Nut & Bolt"
Company locally, or any mill supply house. Ask for "Grade 8" in
whatever style you need. Be prepared to spend more money than at
Lowes, but depending on what and where and how many you buy, the best
Holo-chrome's might not be a lot more bucks. Personally, I find that
Allen-head bolts are less likely to break than cap screws. Not sure
you can buy "machine screws" in Grade 8, but purchased from a good
supplier they will be at least Grade 5.
Twice (at least) in previous replies you mentioned using stainless
bolts rather than the zinc-plated machine screws used at present. You
should note they S/S are NOT stronger that good steel bolts. In
fact, may be quite a bit softer if they are not also Grade 8 or
better. "Stainless" is a quality of the material, and not directly
related to strength or holding/breaking capacity.
I was taught to use Grade 5 or better bolts in the automotive world.
Since I'm using more and more ACQ treated lumber, the need for
non-corroding hardware has led me to stainless research. McMaster has
half a dozen stainless alloys and their differences astounded me.
From their comparison chart + delving into the individuals:
17-4 PH Stainless Steel, 140,000 psi
Combines the high-strength of alloy with corrosion resistance of
18-8 Stainless Steel, 70,000 psi
Offers excellent corrosion resistance; it may be mildly magnetic.
Alloy 20 Stainless Steel, 80,000 psi
Also known as Carpenter 20, it's nonmagnetic and especially
resistant to stress corrosion.
300 Stainless Steel, 80,000 psi
Meet more stringent specifications such as military
specifications. All are passivated (a nitric acid treatment that
creates a passive film to protect the stainless steel from oxidation
316 Stainless Steel, 80,000 psi
Offers even better corrosion than 18-8 stainless steel. It
contains molybdenum, which increases resistance to chlorides and
sulfates. It may be mildly magnetic.
450 Stainless Steel, 170,000 psi
Offers more corrosion resistance than alloy steel.
Unrated hardware, Class 2A, has a 60,000 psi strength.
Grade 5 with a zinc plating is double the strength at 120,000 psi.
If you want strong, buy HoloKrome screws. I believe J&L is in the UK.
Try a company like Fastenall for good screws.