Join Date: Jul 2010
Location: East Central Illinois
DH and I have introduced gunfire to round about 15 horses over the years. The VERY BEST way is to have a totally broken-to-gunfire horse to accompany your mare and give her confidence, and this is what the US Cavalry did, post CW. (In fact, I always try to train an inexperienced horse with an experienced one, because they train in faster.)
Understanding your situation, I would advise the following. Get a cap gun. Your horse will master a smaller sound AND, unlike a whip, she will associate a visual gun with the sound and learn to accept that they are related. Be consistent, but begin by having her in turnout on the OTHER side of a fence, eating her grain--it needs to be grain/treats and NOT hay. Walk away, maybe 15-20 ft. To start with, keep walking, say, "Ready, Aim, Fire" --I'm not joking, and the Infantry DID use those terms--then fire one shot. It's like a pre-cue, and will give her an opportunity to DECIDE to be brave. Stand, repeat the phrase and fire, then do it a 3rd time. She will respond best with YOU between the loud noise and her. If she startles, she won't be a small confined space like her stall, and she WILL want to return to eat.
Gradually reduce to distance until you are firing away from her but situated right over the fence.
After that, you can closer and work with her and the capgun in a round pen, until you are shooting it right next to her, and you are satisfied that she is desensitized to the capgun. IF you are able to use live rounds, you need to start all over, because horses feel the reverbation of heavy gunfire through their hooves, but it will translate.
Just like anything else that is frightening, she will take several months of this to be, as you might say, "bullet-proof." Don't scimp on the time spent. It is well worth it.
As an example, when my Arab and my QH were about 20 years old, and had been reenacting for about 14 years, some bozo decided to double-load a cannon for a demonstration. My horses were about 30 feet from that cannon. I'm sure he wanted them to react for the crowd, so he fired it, double-loaded numerous times. My 2 guys totally ignored it, and ate through his demonstration.
Please let us know your progress. =D