There's basically 3 paths you can follow.
Option 1. Ear Muffs. These have to be suitable to fit under your helmet, a suitable colour, and provide sufficient protection. Typical ear-muffs usually have a NRR (Noise Reduction Rating) of 20-25. There is a set I had on ship that was from "Bilsom" that had a NRR of 28, but there was no way they'd fit under a helmet. Since you have a limitation of fitting yours under a helmet, your choices are slimmed down some, and because they're slim, they'll be even less effective - so a NRR closer to 20.
Option 2. Plugs. There's a wide variety of these that exist, from the multi-baffle yellow ones, to the orange bell shaped ones. The best ones I could find on the market were the "Leight Max-33's, which are the orange bell shaped foamies. They had a NRR of 33, and in my experience did a great job. If you want, you can also get custom molded ones, specifically fit to your ears. I had a set years ago, but lost them, unfortunately.
Option 3. Active/Electronic. There are many options in the Active/Electronic, and it's a question of price, suitability, and preference. The Peltor Tac-6S is the lower end of the line as a set of muffs, and have a NRR of 20. There's the Peltor Com-Tacs which have a 'over the top, or behind the head' option for the headband, and can integrate with Radios and boom mics too - they have a NRR of 21. There are also miniature ones that can be had which can be put in those custom molded in-ear plugs.
If you are going to be staff on a range, and will be on the line for multiple serials, then you may want to double up, adding plugs and muffs. It doesn't give you a complete additive (ie 33 NRR + 20 NRR = 50) but it does give better protection.
Depending on your situation, you may want to reach out to your unit's RQ and see what they have, what they can provide (usually they provide plugs) and what they might suggest you get if you're going to buy some of your own.
In my shooting career, I have had all of the above in my gun-bag at one point or another.