While great for dry fire and manipulation practice, snap caps are not a conclusive test for feeding and ejection.
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I can’t tell you how many times I’ve heard from new builders who load a magazine with snap caps or dummy rounds, cycle them by hand, and then ask why their build isn’t feeding or ejecting correctly. Then they start buying and swapping parts or making changes without firing the gun even once. This is a mistake.
When a semi-automatic firearm loads, fires, and ejects an empty cartridge, there’s a lot going on. The bolt is moving at high velocity. Higher velocity than you can copy with your hand.
When feeding, a live cartridge is long and heavy. It has more of it’s weight toward the front because of the lead bullet.
When extracting and ejecting, an empty cartridge is short and has no bullet or powder in it any more. Most of the weight/mass is toward the back, near the heavy base of the brass cartridge case. As the rearward-traveling empty cartridge is stopped by the ejector and pivots around the extractor, the center of gravity/balance point of the empty cartridge is very important to get the empty to fling out through the ejection port.
The Problem with Snap Caps:
A snap cap or dummy round is usually made of aluminum or plastic, so it’s very light weight compared to a live cartridge. Its balance point is a little further back from a live cartridge, but it’s not too far off. Most of the time they give a fair assessment of feeding, but not all the time.
Ejection is a bigger problem. Compared to an empty cartrdige, the snap cap is much longer, because it simulates having a bullet still in in the case. The balance is also much further forward. They simply won’t eject the same as an empty brass case. Sometimes they won’t eject at all.
Here’s the picture of an empty case, snap cap, dummy round, and live cartridge balanced on the edge of a carpenter’s level. The red line indicates the balance point. Note how far forward the empty case sits. The balance point is way back toward the base.
If snap caps won’t load or eject properly, it may simply mean that the build doesn’t load or eject “snap caps” properly. That’s all! In actual live fire use, it may work just fine.
The real test of any build is live fire with standard factory ammunition. Ignore the snap caps and test the firearm with live ammo before making any changes to fix potential problems. You may find that the “problems” you thought would happen don’t actually exist at all!
If you use the right parts, and follow the 4-step guide to make (almost) any AR9 run 100%, a build has a very good chance of running right the first time, every time. No snap caps needed.
What are snap caps good for?
I find that snap caps are good for dry fire practice, manipulation practice, and for jam remediation practice. I like to stick a few dummy rounds in random places in my range practice magazines and practice my “click, no bang” remediation. It adds a little variety and helps with muscle memory in case a jam occurs during a critical situation.
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