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Big Bore Airguns

Posted by on March 24, 2014

The sport of airgun hunting has been growing at a rapid pace in recent years, and hunting larger game with big bore air rifles is a niche within the market that has seen a surge. There are more guns being offered in .30 to .50 caliber than at any time in the past. In my opinion the interest and demand has grown as hunters look for more challenge (as with bow hunting), more states have added or expanded regulations that allow predators and game animals to be taken, which has in turn led to more custom builders and airgun companies manufacturing products to meet the demand.

These are two big bore carbine "Bush guns" in .451 and .457 built by DAQ and BMFG airguns. They represent what I like in my big bore hunting guns 250+ fppe, two full power shots, small, light, compact!

These are two big bore carbine “Bush guns” in .451 and .457 built by DAQ and BMFG airguns. They represent what I like in my big bore hunting guns 250+ fppe, two full power shots, small, light, compact!

Let’s take a minute to drill down on what constitutes a big bore airgun and what they are capable of doing. While big bore airguns go back a long way, standard production airgun fare has been .177, .20,.22, and .25, which made the .25 the king of the mountain. This was followed by guns in .308 and larger calibers that started to pop up on the air gunning radar screen with some regularity, and today there are more than a half dozen manufacturers offering big bore rifles. The common usage of “big bore” refers to airguns .30 caliber on up, however with the recent introuction of rifles in the .30 and .35 caliber designed to shoot Diabolo style pellets, I like to use the term midbore even if this gets a bit fuzzy with the higher powered .357 shooting cast slugs.

Building a big bore airgun requires more than simply rebarreling a standard rifle; the power plant has to be up to the job. Consider that unlike a firearm all of the energy in an air rifle is stored in, and released by, the gun and not the cartridge. The only power plant capable of generating the energy required to propel a 120 grain .308 bullet at 800-900 feet per second (fps) is a precharged pneumatic. There are many great spring piston hunting guns in the standard calibers, but this power plant cannot generate the energy needed to work effectively with heavier large caliber projectiles. In fact, even when a precharged pneumatic power plant (PCP) is used it requires modification; the volume of air through the valve must be increased, the transfer port which serves as a conduit for air flow from the reservoir to the rifles breech must be opened up, and it may be necessary to increase the fill pressure in conjunction with fine tuning the hammer and valve springs to optimize both ballistic performance and shot capacity of the gun. My more powerful big bores will charge up to 3500 or even 4000 psi.

Sighting in our mid and big bores before hunting in South Africa.

Sighting in our mid and big bores before hunting in South Africa.

The number of shots delivered from a single fill comes down to the volume of air contained in the guns onboard reservoir and the volume of air / pressure needed to drive the projectile at the desired velocity. The gun can be tuned to provide the desired tradeoff between power and shot count. The physical size of the reservoir is limited in that you don’t want the diameter so large that it precludes an ergonomic forestock , or the length to exceed that of the barrel. I would expect to see a bottle forward big bore design to come a long at some point in the not too distant future, and is already being used in some mid bores such as the FX Boss. .30.

Earlier I stated that there is more to building a big bore than simply adding a bigger barrel, which is not meant to imply that the barrel selection isn’t a key design factor. The twist rate of the barrel used in an air rifle is generally slower than that of a firearm to take into account the lower velocities generated. The lands and grooves are also cut shallower as they do not need to imprint the bullet to impart the optimal spin. As to barrel length, it is true that the longer the barrel the higher the velocity (to a point). However, an exceedingly long barrel is more difficult to maneuver in the field, and at the ranges I like to shoot (50-70 yards) has very little impact with respect to terminal performance. I know my views on this are not shared by all in the big bore hunting community, but I think trying to go long range with an air rifle negates the reason most of us want to use them, mainly to up the challange.

The desired performance profile depends on how the rifle will be used. For predators at closer ranges n(say 50-60 yards) I have been using (and liking) the midbores. If reaching out further my preference is for a flat shooting .308 or .357 sending a 100 grain bullet down range at 900 fps. With this set up I’ve got a coyote gun that will reach out to 100 yards, it can stretch further but efficacy will start to fall off. For larger animals such as deer and hogs I like a larger caliber such as a .457 doing 750 fps. The critical point with a big bore airgun is that it must be accurate; certainly it should group under an inch at the maximum distance you plan to hunt. The guns I shoot are all capable of printing clover leaf groups when sighted in at 50, 75, or 100 yards.

A flat shooting .357 can be deadly on coyotes out to 100 yards. Flat shooting and hard hitting with a 120 grain cast bullet.

A flat shooting .357 can be deadly on coyotes out to 100 yards. Flat shooting and hard hitting with a 120 grain cast bullet.

In terms of power, it of course depends on what is being hunted. As a general rule of thumb I want a gun that generates at least 125 fpe for long range (100 yard) predator hunting. This will allow complete pass through on a coyote shot broadside in most cases. For hunting larger game such as hog and deer, I think a 220 fpe minimum is appropriate for a 50 yard hunting gun. Boost the power and you can reach out a bit further, however as with bow hunting, the airgun hunter needs to exercise a bit more self discipline when it comes to shot selection and pass on longer shots. With respect to the conventional wisdom stating that 1000 fpe is needed for deer sized game with a firearm, field experience clearly demonstrates this is not the case with a big bore airgun. With these guns the killing ability is purely a product of punching a big hole in the right place, which these slow moving heavy projectiles excel at. Anything from a .308 to a .50 caliber projectile at 250 fpe is going to produce complete pass through on even a big hog at 50 yards in most cases.

I can report that there is a lot of activity out there, with several new mid bores and big bores being developed. I would predict that within the next year we will see close to a half dozen new mid/big bore rifles coming to market. The take away is that if you’ve ever wanted to get into big bore air gunning, now is a great time to give it a go. There are more places to hunt with more legal species, and more guns and projectiles to choose from, than ever before. Over the years I’ve taken a lot of firearm hunters out for their first big bore airgun hunts, and in almost every case I’ve seen the light switch on as they experience the excitement of using these unusual yet very effective rifles in the field.

YouTube    https://www.youtube.com/echochapman

Website      http://www.americanairgunhunter.com

2 Responses to Big Bore Airguns

  1. Scott Dellinger

    Good article Jim with great info. Hope to talk to you about a big bore soon. Scott

  2. Keith

    Jim Chapman I enjoyed reading your expert information on big bore air rifles, I was wondering what scope and scope mount was installed on the Sam Yang Dragon Claw .50 cailber. I recently purchased one and needed some advice. Thanks

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