I'm the coach for a high school air rifle team.

My shooters are having a problem with the sling slipping.  The nylon the sporter slings are made of is pretty slick, you need to either crank it down so far that fingers turn blue, or have it constantly slip down the arm.  

I've had everyone pin the sling to their sweatshirts, but 80% of the time, it works 20% of the time.  Not really effective.

Rule 4.3.3 says "Top Grip™ or a similar material may be stitched to the inside of the arm loop, but the lining may not be so thick as to provide padding."

Rather than Top Grip (I hate--because I can't--sewing), I was thinking something like the dots used as grippers on gloves--something that doesn't have to get stitched in.  

Two questions--A), is that legal, B) what do I use?  Silicone caulking is actually too slick (tried it), so I need something I can dot/smear onto the sling and that isn't slick.

Contact adhesive comes to mind first.  Not the stuff for paper, but the industrial 3M or similar stuff.  Just coat the inside of the arm loop with two coats and let dry completely.  It should stick to the webbing, won't be thick, and will be substantially more sticky.  Even the top grip could be glued on instead of stitched on with the contact adhesive as well.  There, you just coat both mating surfaces, and stick together after it drys enough.  Follow the directions on the container.

Put some little dots of hot glue on it and let it cool.  It will be a lot more grippy.

Or you could get some plasti-dip in a spray can and try that.

When I shot 3-position sporter air rifle in high school, we just used the old OD cotton M14 slings and they worked very well. Tightened the rear (bottom) loop around the bicep (we omitted the GI swivel hook) and the front had an Uncle Mike QD swivel added.

They didn't slip during a match once tightened initially. They're also inexpensive. Try to find original USGI though, since Chinese copies have inferior keepers that can open up on you (or they did last time I wasted $10 on one several years ago).

Know very little about rules and it sounds like you need DIY.

Boonie packer slings (made local here) are great minimalist slings with good grip.
Also, how about some adhesive neoprene material? I just bought some on eBay for soft bedding a Beeman R7, 1/8" thick.
Other idea? Shelf liner or carpet non-slip under pad.

Good luck.

Anyone have experience with slings like this? https://www.hataservices.com/sporter-air-rifle-slings.html
(found on Google)

Looks like current rules also allow for a button to be attached to the arm or shoulder in order to prevent the sling from slipping.
We didn't have buttons, but that sounds like a great idea!

Also see: http://www.odcmp.org/1002/inc_howto.asp

Yeah, I'm thinking something like plasti-dip, truck bed liner, or rubberized undercoating.

Rubber cement is quite grippy.

You could also brush on Plastic Coat, but it might be too slick once cured.

Anything in rules about using double-sided stick tapes?

I'd make the team new slings but there may be rules about US only accessories

A possible fix: Get some neoprene based glue, and a few old rubber inner tubes. Cut the tubes into strips about 12" long by whatever the sling width is. Glue strips on the inner surface where the sling is in contact with the arm. Should be good to go.....I put a similar rubber strip in the Swedish sniper slings I make, but that stuff is more like the sort of rubber seen in Ikea as drawer liners

Some really good ideas, here.  Thanks, they're all better than what I could come up with.

We do the safety pin thing, and it works, but it's not enough.  It winds up pulling the safety pin down as the sling slips.  So, looking for something else to increase the friction.

M1/M14 slings are going for $25 each right now... and I've got 18 shooters on the team.  

They've all got parents, right?

My mom used to have to pay for a bunch of school supplies, because it was mandatory in public school.

As far as extracurricular activities like MCJROTC, any needed items the Marine Corps couldn't/wouldn't provide were purchased individually by team members, their parents, or all the parents and instructors would get together and organize a bake sale or something. Occasionally just send cadets out requesting donations (although that was the least-favored method).

I think the button concept would work well if you've got anyone good at sewing. You'd need a large high-quality button and good strong thread.