Haven't seen this asked before so I'm asking:

If I wanted to sell a 15 round mag with a gun, what would I or a gunsmith have to do to make the magazine "legal?" that is reduce it to 10 rounds.

How can higher cap magazines be altered with a new end cap, something welded/epoxied/etc. in to make it conform. Or, if a gun with a 15 round mag was sent to my FFL from out of state, what could a gunsmith do to make it 10-round legal before I took possession of the gun and mag.

In other words, is there anything that can convert a higher cap magazine down to a legal one? (And if there is, is this a business opportunity for some enterprising ffl gunsmith. I'd pay a reasonable price to get a mag "fixed" if it meant I could sell (or buy) a gun without having to go on the open market to try to find a 10-rounder to fit a particular gun.)

Several problems or issues with said modifications.

1) easily convertible back to above 10 rounds
2) cost. Alteration or modification would probably cost more than just buying said 10 round magazine or at least 1/2 to 3/4 the original cost. So would make it pretty non feasible - cost wise.
3) availability - hard to find or non existent or not available to all mags.
4) intent. If for personal use. The law allows if you had said mags - they are good to go for you. If for sale you are reaching into grey area and would potentially be putting yourself out there. Not worth it.
5) if one is a FFL - sell out of state and make potential profit money? I mean it’s such a small amount unless one gets it for free. Mailing/shipping cost. Finding a buyer etc. lots of energy to make this deal happen.

Good try

Don't Nueter a standard cap mag, just buy 10 rd ones. In some cases for obscure firearms that may not be possible though.

In almost all jurisdictions a rivet into the mag that prevents the follower from exceeding 10 rounds is acceptable.

Consult a lawyer for certainty however.

If someone is looking for a business opportunity I guess you could manufacture followers with a post that limits it to 10 for popular magazines like AR, Glock and p320.

If someone wants to convert them back it would be on them to get a standard follower and swap? Without the standard follower it would be permanent. The law would prevent the buyer from acquiring the parts to make it standard again, unless the buyer was out of state.