There are a few in this circle, on the ASSRA forum that are playing and competing in this game so they’re familiar with it.  We’re not shooting bottle caps off your favorite beverage; it’s the caps or a facsimile of those little cardboard discs that sealed the milk bottles in days of old.  The discs measured about 1.6” in diameter and they were about .040” thick.   Milk isn’t delivered in those bottles to your doorstep in most places any longer so it makes the discs tough to come by.   Bear with me as I explain some of our processes and evolution of the game, it may take a couple days to get everything  posted.  
We have a few years of shooting history in this game and a write-up has been on my to do list for a while now.   I’ve contemplated an article for the Journal and that may still happen but at the speed I operate, a few more years may pass before it’s accomplished.   A post on the forum is easier and there’s a chance I’ll get a response or two… and as a member here refers, I'll “hook” some more.
The game as we play it locally is as follows.  A shooter is given 39 discs and they are responsible for marking the center of each disc.  This is all the shooter’s responsibility, mark the center exact… A commonly used stamp is about ¼” diameter, with a ring or a dot in the very center.  The disc holder usually has a hole in the center with minimal clearance to the stamp itself.  A simple ink pad provides the medium for the stamp.  Once the discs are marked, 13 are attached to a sheet of paper via double stick tape, a special board with headed screws, or a 3D printed holder, then set up down range. The range we shoot caps at is 50 feet indoors.  We shoot from benches, tables, cross sticks, even prone position.  Many of our matches end up with several shooters so we shoot a single card at a time in relay fashion.  In most cases, we run a timer that allows 10 minutes to shoot a card, some clubs go more, some don’t care about the time constraints (some of our shooters need a shove and a few need to realize this isn’t a sprint, so the clock paces both types).

The measurement is recorded immediately, if not and the disc is removed from the platen, it’s almost certain a different dimension will be measured, generally not favorable on the second attempt.  In our rules, we shoot 13 discs on a card, this gives the shooter a mulligan of sorts.  You are allowed to throw one out, make it your worst one and you total up the runout on the remaining 12 discs.  This is the score for your card. 
In the past five years, we’ve made some changes to the bottle cap gear.  The original discs were 1.6+ inches in diameter.  Not something common off the shelf.  I made a few punches that fit a RCBS reloading press and after we cut photo backing into strips, we were able to run this material through the process and produce a duplicate cap like the originals.  It was a lot of tedious effort.  A half dozen local folks here would borrow a punch, go to craft store and buy enough card stock for several thousand discs and spend a couple weeks of evenings doing this… I’m not sure about them, but I’m over it.   
On a whim, my son asked if there was a problem changing the diameter of this disc, I couldn’t see an issue so I asked why’s that?  He said the .040” thickness was within a few tenths of a thousandths of 1mm and there were people selling game chits on eBay that measured 1mm thick, but they were 1” diameter…  We’ve discovered, if you can’t keep your hits under the .125” measurements, you won’t make middle of the pack so they should work fine.  I had made a few of the measuring devices from scraps of steel in our shop and it would require some effort but I could make some new platens for the smaller discs and we’d be in business.  The majority of the chits we’ve discovered have a measurable variation of a couple thousandths of an inch on the diameter, in some cases it benefits the shooter and in other cases it hurts them, in the end, it’s a wash.

We pondered what rifles we’d allow, what scopes, etc. etc.   It’s a game that challenges equipment, ammo and probably most, the shooter.  In the last five years that I’ve been shooting caps, I’ve discovered it’s not about whether it’s vintage or modern, it’s about how the equipment is put together and who’s doing it.  We’ve seen folks show up with extreme bench rest rigs, best barrels, best actions, best glass, $$$$ ammo, and barely make middle of the pack.  An individual on this forum shooting a 100+ year old rifle, built around this game, shoots and when he doesn’t get in the way of the rifle, quite often shoots better than the modern whiz bang rifle does.  I put a new barrel on a rifle this last week and went to the range and shot it last night for the first time.  This one needs a little tweaking yet but it shows some serious potential.  Up until this one, my goto rifles have been a EJ Ballard and a bolt action single shot I built.  So our rules are, anything goes.  Bring out your best, put on your game face, and try it.

"The range we shoot caps at is 50 feet indoors.  We shoot from benches, tables, cross sticks, even prone position"

The different indoor galleries back in the day shot it at their gallery distances, these varied from 50' to 75' from what I've read.

The latest range a few of the original folks here shoot at wasn't marked very well and nobody paid much attention to it until we started going to other ranges.  We were all adjusting our sights, never said much to each other until one day a shooter came in with his range finder and determined that for about a year we'd been shooting at 60'.  Complacency bites every time.  We adjusted our shooting line and when we travel now, there's very little sight adjustment...

We have a group that has went down the (GT) rabbit hole and the koolaid seems to have worked, they really are having fun being able to shoot 50 foot bench that can kick your butt if you don’t do your part. Nice thing is you get to work on your bench technique without the normal outdoor winds, and during the winter. That said you can still have mirage and airflow that can surprise you. Try it at your own risk!

The bird is starting to sing. Looks like the new barrel is working

This looks like fun. Now to take it down to .177 for 10 meter air rifle in back yard. Hummmmmm. Wish I was a lot younger.
Flatlander