I would suggest contacting someone that has specific knowledge of the noise hazard, guidelines regarding acceptable levels of noise and the required minimum protection. Please don't risk hearing damage. In industry I always have access to health and safety staff and hearing protection falls within their responsibilities. I no longer know how things work in DND but I do recommend you consult someone with the necessary knowledge. They should be able to tell you what minimum level of protection is required.
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.
Having just completed the mandatory CAF Safety Trg on the DLN, I can state that you are not permitted to purchase your own safety gear. You must use what the CAF provides. If you don’t think it is adequate, you can write a UCR and state why. UCRs are ridiculously easy to write, so it is surprising so few people take the time to do it.
This is specifically for on a range, hence the need for it to fit under a helmet. We're issued 3M foam ear plugs, at least on the ranges around here, and I was thinking of doubling up with plugs underneath.
Ok, so this is where my experience comes in. It has been decades since I wore a uniform and now I work as an engineer, often at large construction sites.
Everyone on site has a right to a safe workplace. There are health and safety committees established to ensure that worker safety concerns are addressed. If ANY worker identifies a potential safety hazard, it has to be investigated. If, for example, a worker thought that the noise levels were potentially dangerous, in no time a sound measurement expert would be on site, measurements taken and (if required) appropriate hearing protection specified, acquired, provided to all workers and workers REQUIRED to wear that hearing protection at all times in the designated 'noisy' area.
From the above, it sounds to me like there has not been a proper assessment of the noise (volume, frequency, etc.) and subsequent process to identify the appropriate hearing protection, where it is required, etc. I would strongly suggest you take this up with whoever is responsible for safety where you work. This is YOUR hearing at stake and this should not be taken lightly. You should not just have to assume that you have appropriate ear protection - you should KNOW what is required and that the protection is appropriate for the noise hazard.
The OP said it was for the range and there has been lots of testing for the noise level on the range and direction issued in "Training Safety" for what type of hearing protection is the minimum required. In addition, all participants on the range should be given a safety brief which would remind all pers to wear ear protection (and where to get it if they don't have it). plus the ARSOs and other range staff should be checking to make sure hearing protection is being worn at all times.
I offer you my advice as a sufferer of tinnitus and hearing loss, partly (mostly) as a result of my CAF career.
No, you are not allowed to purchase your own PPE.
No, I have never been prevented from wearing my own hearing protection on the range, as long as it's in addition to, or better than what is on offer.
I wish, usually at night when I'm trying to sleep, that I had been smarter about wearing PPE properly and in all circumstances when I should have been wearing it, but didn't for whatever reason. Younger, dumber me didn't wear it or didn't wear it properly for fear of misunderstanding range commands etc. I recommend high quality electronic muffs that amplify things like range commands, worn over either foam plugs or custom fit shooting ear plugs. It will cost you a good chunk of money, and from where I sit be worth every cent you spend.