If you're using the rational of the insurance co. of not alerting thieves to a high value package, then registered mail is not what you'd want. The minimum insurance I believe is $650 and the whole idea of reg mail is to move high value packages.

IF on the other hand,,,you want maximum security in transit through the USPO,,then reg Mail is what you want to use. It is slow, as pointed out.

Personally I'd still send it overnite UPS or FedEx. The less handling the better. The longer in transit, the better chance of damage.
Take alot of digital pics of the gun before wraping it for ins purposes including ser#.

I've used both Overnite & RegMail and have never had any problems with either in 40 years.
Have had a couple of problems when shipments delivered by Ground service.

Everyone has their own favorites and their personal horror stories with shipping. Pick one that you're comfortable with and go with it.
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Most PO's won't provide the Reg mail tape for you anymore. They even took the clear tape dispensers off the counters in most locations. You have to buy tape there if you want to tape up a package. Not all of them have the paper RegMail tape for sale. I'm sure there are a few around that still provide the service but they are getting few and far between unfortunetly.