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12 ga Shotgun Minishells by Aguila®™

Modification & Testing of Mossburg®™ 500A 12 ga Pump Shotgun

Aguila Ammunition - Quality, Performance, Innovation

12 ga Shotgun Minishells - Manufacturer Data

The Above information comes from the manufacturer. Please see their site for more information.

Reloading at best is a 1 second lag in firepower. That speed is based on a lot of practice. Reloading, is the time to advance. A person can advance 21 feet in 1 second......42 feet in 2 seconds.

The moral to the story...You should always bring more than 1 gun to a gun fight. The Military brings martial arts, knife, handgun, and a  rifle. You see the difference.  You can never have enough ammo or backup.

The idea for the modification came from being to cheap to purchase a 10 rd Mossberg pump 590 aka military/police setup for two matches a year.

MY MOSSBURG 500A Modification  

aka - Mossberg Mini Plug


You assume all risks, known and unknown for this modification. This will void your warranty. This could result in miss feeds. Failure to feed has been reported by others but not by using this modification. I did not have any failure to feed during testing and shooting in matches. Additional testing by independent sources needs to be performed as to FTF.

I compete with my Mossburg 500 in USPSA Shotgun matches. These matches usually have stages with following characteristics:

Points divided by time so you cannot dilly dally.  Reloads are the largest time factor. With the modification I can carry more in the tube. Using regular or magnum 12 ga shells  the tube magazine holds five shells and one in the chamber, giving a total of 6.


Steel knockdown targets ranging from 8 inch diameter steel plates, to larger 4 foot size knockdown steel. There are stands that hold regular clay pidgins and mini pidgins. These could be as close as 10 feet and as far away as 50 feet.    


Cardboard regular USPSA target and / or USPSA 1/3 Target set at 40 to 60 feet. Scored based on pellets x Score on A-D ring


Cardboard regular USPSA target from 20 feet to 100 feet . Scored based on A-D ring

Photo of Regular Rifled Slug compared to Aguila mini slug


A small plastic/ nylon spacer attached to receiver with set screws.  The Mossberg-Mini-plug was designed out of nylon. Easy to cut and mold with a dremel tool. Then attached to the Mossberg receiver with two self tapping screws going through 2 undersized holes in the receiver, which in turn taps into the Mossberg-Mini-plug.

With the spacer in place you can still load regular size shells one by one if needed. This is a removable feature. Requires 2 holes drilled through the receiver. Once in place everything performs in real world situations. The matches I have used the mini shells have a total round count of at least 60 rounds. Way more ammo then you will ever use in a home defense situation.

Like any modification, it will void your warranty. I will not be held responsible for your modification.

Dimensions for the Mossberg-Mini-plug.   

Picture of Mossberg-Mini-Plug


Pictures in receiver



    First Test was a basic cycle test. I used a foam block with heavy metal foil tape as a prototype.

    Cycle test with nylon Mossberg mini plug passed with flying colors with 200 rounds cycled.


    1- Cycle Test.  Holding shotgun down, up, and at different angles.

    2- Shoot 10 shots total waiting 15 sec between each shot.

    3- Shoot 10 shots total rapid as possible.

    4- Shoot 10 shots total rapid as possible, mix shot, slug, then reload 10 and shoot rapid as possible.

    5- Shoot 15yds shot for pattern test twice

    6- Shoot 25 yds slug for analysis using Modified cylinder and rifled choke

All tests passed.  No misfire of any kind. No miss feeds due to  Mossberg-Mini-plug

Pictures of 25 yard test of the Slug, below    

Pic 1a.

25 Yard Sandbag rest

Mossburg Shotgun 12 ga Pump

Modified Choke

Wolf 12 ga slug Standard size.

Not too bad of a group 

3.5 Inches.













Pic 1b.

25 Yard Sandbag rest

Mossburg Shotgun 12 ga Pump

Modified Choke

Aguila Minishell Slug

Seems to open up a bit.

11 inch group.

For personal defense works fine out to 25 yards.

But for the long shots in USPSA there is a margin of error.











Pic 1c.

25 Yard Sandbag rest

Mossburg Shotgun 12 ga Pump

Rifled Choke

Aguila Minishell Slug

Seems to tighten up a bit.

 to 9 inch group.

For personal defense works fine out to 25 yards.

But for the long shots in USPSA there is a margin of error.











I came out to the range to shoot a shotgun match. Everyone took bets from;  It won't have enough power to knock down the steel, ha ha ha...  Sure they work? it will jam just watch ...etc. 

Time to put up or Shut them UP!.   In a quick test in front of the Range Officer, crack-a-towa- the Minishell shot did have the power to take down the steel.

The Minishells shot worked great all match.  The slugs have no stabilizers , ie. rifle grooves. so once you get out to a distance the slugs tend to spread a good bit compared to regular rifled slugs. I even tried a rifled choke to add some spin.  In a match I missed , or had a low D ring score on the targets at 80 feet. I did some tests and took photographs. The difference is obvious.

A regular size shell can be inserted as needed one by one. For example add a regular size rifled slug for the far away shots.

I no longer use the mini shells in matches due to the new rules on power factor. The matches I did shoot showed the modification worked in the heat, dust, repetitive firings, and staged scenarios of those days.   



The testing and matches proved the rounds are manufactured well and offer reliability.

There are limitations for the Slugs, which come into play once you get past 60 to 75 feet range.

The difference I found was the diameter of the slug. Regular slugs fit tighter in the barrel. The factory

might be able to improve this.  25 yards is well within a practical home defense parameters.

The ability to load a regular rounds is a plus. Especially for a 100 yard shot.


I personally opt for more ammunition. In this case 40% more for a total of 10 rounds at the same weight as 6 rounds.

The rounds are 30 to 40% less powerful, less recoil, still effective at home defense ranges.


This set up could work for Military or Police.

So take a look at your shotgun, and if it is a pump, you may be able to make the modifications and up your on board firepower by 40%.

The Aguila site has information on other shotgun modifications and an actual Shotgun designed from the ground up for the mini shell.   






Shotgun Shell type Comparison  
By USPSA Power Factor      
Min Power Factor to knock down steel is 125  
Aguila Mini Shot      Reg 3 Dr Shot  
FPS speed 1175   FPS speed 1145
Projectile Weight 277.78   Projectile Weight 492.00
Power Factor 326.39 57.94% Power Factor 563.34
Aguila Mini Slug      Reg slug  
FPS speed 1250   FPS speed 1600
Projectile Weight 385.808   Projectile Weight 438
Power Factor 482.26 68.82% Power Factor 700.8
Aguila Mini Buck      Reg OO Buck  
FPS speed 1200   FPS speed 1200
Projectile Weight 304.2   Projectile Weight 484.2
Power Factor 365.04 62.83% Power Factor 581.04
Buck Shot Pkg size wt grains  
four 1buck rounds 0.3 40    
seven 4buck rounds 0.24 20.6    
Total Wt grains   304.2    
RIFLE Round        
7.62x39 123g     5.56  
FPS speed 2400   FPS speed 3100
Projectile Weight 123   Projectile Weight 55
Power Factor 295.2   Power Factor 170.5
HANDGUN Round        
.45 cal HG     9mm cal HG  
FPS speed 920   FPS speed 1125
Projectile Wt 230   Projectile Wt 124
Power Factor 211.6   Power Factor 139.5




Mossberg, 500A, 590 Mossburg®™  O.F. Mossberg & Sons, Inc. - Firearms, Shotguns, Rifles

Aguila®™ Ammunition                      Aguila Ammunition / Centurion Ordnance, Inc.

Wolf®™ Ammunition                         WOLF Performance Ammunition 

Federal®™ Ammunition                    Federal Premium ammunition

Firearms Tactical©                   


The photos are copyright © 2008 All Rights Reserved William E. Fowler, AceEyeEd

Article copyright © William E. Fowler 2008 

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Updated on 5/15/2009 

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