I suggest you to use fo4edit tool - easy and convenient way of modding and patching.

A. Plugin work:
1. Load weapon and ACF plugins
2. Look for weapon entry in weapon plugin and override it to new plugin(suggest .esp with esl flag) - name patch plugin as you want.
3. Right click on patch and choose "Add Masters" - add ACF as master
4. Look for weapon template in ACF, select both ACF example and target weapon entry - right clock - compare
5. Drag and drop to target entry VMAD(script reference), ma_ACF keyword, and attach points which you'll use (here present left and rear mounted ammo counter attach points - choose only if it'll be used)
6. In VMAD - look for TestWPN reference and change it with weapon FormID

If you want to use on-sight ammo count projector for sights or other purposes, do this
A-2(optional) Additional attach points:
1. Override Object Modification entries related to sights which you want to use with on-sight ammo count projector to the patch plugin
2. Add specific attach point - you can see how it was done in Sight Template in ACF plugin in Object Modification tab

Save your patch plugin. And find it location if you want to change its place.

B. Nif work:
1. Unpack from BA2(Archive2 tool provided with Creation Kit is a good choice) receiver (and sights - if optional) nif model, open with NifSkope
2. Unpack ammo counter model from ACF archives, open with NifSkope too
3. Create new attach point in BSConnectPoint::Parents of receiver nif model - increace Num Connect Points value by one and press refresh button(two green arrows in circle)
4. Locate this new point - clear Parent name, add to Name specific name(P-AmmoCounterLeft, P-AmmoCounterRear, P-AmmoCounterSight)
5. Copy Node with ammo counter model inside main node and paste to receiver nif(right click - Block - Copy/Paste Branch, or Ctrl+C/V while selected in left window)
6. In receiver nif, right click on node with ammo counter model - right click - Transfrom - Edit - and find a new place where ammo counter should be placed - Accept
7. Again - right click - Transform - Copy - to copy final position
8. Go to BSConnectPoint::Parents - to new connect point - right lick on place with colored lines - Transform - Paste. Remember - if you changed rotation angles and scale - be sure they're were copied - if not - add this values manually
9. Remove node with ammo counter model, save receiver nif file

B-2(optional) On-Sight attach points:
Work same as B part, but require some extra work
1. Mostly, sight nifs didn't contain BSConnectPoint::Parents entry - you need to create it by yourself - Right click - Block - Inster - Bethesda... - BSConnectPoint::Parents (or copy from Receiver nif - change Num Connect Points to 1 -  refresh - create new attach point)
2. Possibly it wasn't created inside main Node but along with it - copy and insert inside node - remove old BSConnectPoint::Parents.
3. Create attach point like in B

And save, don't forget.