I tried something last night that worked well for me, but use at your own risk.
There is a program out there called F-Droid that will allow you to transfer apps between devices. In this case, I had a 10 year old Samsung tablet running Android 4, that had Strelok Pro on it. This tablet stayed in my range bag and was only used for Strelok Pro and taking notes. It would no longer connect to Google Play so I don't know when the last time it updated.
My goal was to see if I could get Pro over to an LG tablet that was a little newer, running Android 5. Downloading an older compatible version of F-Droid to both tablets, and using the built-in Bluetooth functionality, I was able to share the version of Strelok Pro in the Samsung tablet with the LG tablet, and install it.
When finished, I deleted F-Droid from both devices and Strelok Pro continues to run on both of them.
Of course the bullet database, reticles, etc are not getting updated but the Bluetooth functionality for external devices, as well as the weather capturing feature, still work fine.