Seems like a rather long time to risk storing firearms. I can think of half a dozen ways the firearms could be gone. One potential problem is if you stopped making payments to the storage provider. After a couple of years nonpayment, they will consider the contract breached and the property abandoned. They can then sell it to recoup their storage fees.
If you really need to do this, the best option would be to leave them with a friend or relative. No storage fees (unless you would like to gift one of them as a thank you). Second best option IMO would be to sell everything and buy replacements in 10 years.
I don't know the dollar value of the firearms to be stored, nor if any of them are rare/collectible, have sentimental value, or can never be replaced. Depending on those factors and others, selling and replacing may or may not be an attractive option. Any losses you take on the selling price and the cost to replace them would be offset by the savings in storage fees.
OGC charges $32/month, and I assume that's per firearm. ($32 x 120 months) = $3,840. Then multiply that by the number of guns. The cost to store your guns for that long will likely exceed the value of the firearms, assuming they aren't valued at over $4,000 each.
As for the ammo, it's better to sell it. It's only going to be 10 years older by the time you retrieve it. Any extra money you spend replacing it, based on what you sell it for and what it costs in the future, would be offset by the savings in storage fees. Again, if you want to leave it with a friend or relative, that would be a better option than paying for storage.
These are just a few things to consider. When I first rented a storage locker to clear out some of the stuff from our house, we stored the things we rarely used. After a year, I spent $1,100 for storage rental. Between stuffed animals, holiday decorations and other stuff we didn't use that year, I could have taken that $1,100 and bought what I did use as we needed it. I would have come out even further ahead if I sold what I could instead of hoarding them. If I rent another locker, it'll be for a very short duration and only if the value of the property greatly exceeds the cost of storage.
How much more are you willing to pay to keep the things you already own?