Thanks for your question. The short answer is yes, other polymer plugs are legal but you need to refer to the USPSA rules. I contacted John Amidon, USPSA's rules expert, and asked his input. He's the VP of USPSA and the Director of the National Range Officers Institute. When it comes to USPSA rules he is the final word. Here is what he sent me.
Some of the Jentra plugs are legal, others are not, they have some made of metal and others that extend down below the grip.
From the July 1, 2009 revised Production rules which are posted on the uspsa.org web site under match rules states:
22.5 External plugs (such as Seattle Slug) Remains in effect – external plugs which alter the grip profile or extend past the
bottom of the grip are PROHIBITED.
Also from an NROI ruling in 2008 and is also listed on the web site under match rules then NROI rulings.:
Title: Plastic Plug for Glock's in Production divisionUpdated:5/06/08
Rule number:Appendix D4
Ruling authority:John Amidon
Question Can I now insert the plastic plug in my Glock and will it be legal for Production division?
Ruling The plastic plug that fits flush with the heel of the grip is allowed, like the Jentra plug. Plugs that extend below the heel like the Scherer polymer slug plug or other similar models are not allowed. Metal, brass or the Seattle Slug plugs are not allowed either
Be sure to review the rules carefully and talk to the manufacturer of the plug you plan to use to make sure it conforms to USPSA rules. Most manufacturers know John and will submit products for review or clarification. If the maker you are considering has not, I'd suggest getting them to send the product to John for an official ruling if there is any doubt. You don't want to end up with a DQ in the middle of a match due to a misinterpretation of the rules. Hope that helps. - ERHARDT