Welcome to our Resources Page

Here you will find information that we have put together on subjects other than product owner manuals.  We hope you find it highly informative, and will share the knowledge with others in our community.

If you have any suggestions or comments, please reach out to us with our Contact form!!

How does the Multiple Caliber System work with the AR-15?

Let’s start by explaining a couple things.  First is a quick picture of the anatomy making up a firearms cartridge:

Ammo-Terms

The next picture shows an AR-15 chamber with the cartridge loaded into the chamber.  The two important elements to understand are the chamber, and the bolt.

Chamber

An AR-15 chamber is part of the barrel.  By changing to a new barrel, we can change to a new caliber.

Chamber2

The AR-15 bolt dimensions determine which calibers it can run.  The two dimensions to understand are the Bolt Depth, and the Case Head Diameter.

Bolt

Bolt Depth

We recommend using bolts with the same Bolt Depth (typically .124/.125″) for safety reasons.

Case Head Diameter

This dimension is the most important in determining the calibers you can run with your AR-15.  Some AR-15 bolts can run dozens of calibers, and some can only run a few.  That is mostly due to the popularity of a given cartridge.

AR-15 Bolt Selection Tool

We put together the following AR-15 Bolt Selection Tool, to help you determine which bolt you’ll need to run different calibers in the AR-15 platform.  As you will see, there are 4 primary bolts that run the majority of the rifle calibers in the AR-15.

Additional Considerations

Each pistol caliber listed in our AR-15 Bolt Selection Tool is based off a “blow back” bolt.  These pistol caliber bolts are weighted to the caliber (much like a handgun slide).  We recommend not using a direct impingement bolt to run pistol calibers in an AR-15.  That is because most commercially loaded (factory) ammunition creates fouling that quickly plugs the gas tube.

Another difference between the AR-15 bolts are the extractors.  Each bolt has a slightly different extractor and/or extractor spring.  This is done to better match the extractor function to the shape of the cartridge’s case head.

The Future of AR-15’s

Going through our research, we decided that the AR-15 as a Multiple Caliber platform is not only viable, it is highly beneficial.

To that end, we wanted to help simplify and standardize the multi-caliber rifle.  We started with the bolt to caliber struggle.  How do you ensure you are using the right bolt for the right caliber?  At the range?  Under stress?

A growing number of AR-15 owners are painting and coating their firearms in different colors, so we knew color coding was not going to work.

There are already a multitude of numbers associated with firearms, so number coding would just add to the confusion.

Then we had the answer.  Letter Coding the bolts and barrels.  You’ll notice in our AR-15 Selection Tool, that the bolts are grouped by letters.  A, B, C, D, E, and F (the primarily standardized rifle caliber bolts).

To further standardize and simplify the process, we removed the F bolt group.  The 7.62x39mm bolt is capable of running the two calibers that the F bolt runs.  It also allows us to run the same bolt depth across all of the bolts.  Much safer.

Bolt group E only runs 5.45x39mm, so if you don’t plan on shooting AK-74 ammunition, that leaves you 4 bolts that run all the centerfire rifle calibers for the AR-15.  Easily the large majority of possibilities, with 70+ options.

We hope you find this information to be useful, and ask that you share it with others.  By sharing knowledge with others in the firearms community, we all become better firearms owners.

Please use this information at your own risk.  Frontier Tactical can only confirm and validate the testing we performed under controlled and expert conditions.  We have no control over the mixing and matching of components found in our industry.

We work diligently to provide a safer and more compatible firearms/parts/components industry, and hope you will push for the same.

To this end, we will mark our bolts and barrels with the Letter Coding found in the AR-15 Selection Tool.