The AR-15 is perhaps the most popular carbine in America.  It's commonly compared to the AK-47 despite the vast number of differences between the two weapons.  While AR-15s are modular weapons meaning many of the parts can easily be replaced or upgraded, barrel length is often left unchanged at 16 inches. There are many reasons to deviate from the standard 16" barrel length.  Many M16 rifles were built with a 20" barrel that gave it a little better accuracy.  Some AR rifles are available with 20", 18", and 14.7" barrels are also become popular.  When switch from a 16" barrel to something longer or shorter you will more than likely have to work on the gas system and/or buffer.  This is because once you start messing with barrel length it changes the amount of a pressure in the chamber.  Using different configurations of gas tube lengths or pistons and buffer weights is an effective way to control the recoil and proper cycling of the weapon.  Some manufacturers are even producing gas tubes or upper receivers that allow you to adjust gas pressure.  Gas tuning is also needed once a suppressor is attached the weapon.

Back to barrel length, the main reasons for going shorter or longer are generally weight and accuracy.  A 14.7" barrel is legal if the flash hider is permanently attached (pined and welded).  Counting the length of the flash hider it usually brings the barrel length to 16" or more.  If you plan to use a 14.7 inch barrel without welding the flash hider, you would need to submit a form 1 as it would be less than 16" and deemed a short barrel rifle.

Popular SBR (short barrel rifle) configurations are 11.5", 10.5" and even 7.5"  These configurations can shave a few pounds off the total weight on the weapon.  The only downside is the increased recoil and slightly less accuracy at longer distances.  Choosing a fluted barrel can save even more weight but usually comes with a premium price tag.  If you are considering any of these changes you probably know it won't be cheap.  Customizing an AR-15 which is normally around $1000 for a stock rile, can run well over $3,000. The following photo shows an 11.5 Adam Arms upper with a special buffer and spring.

Remember that if you go with a short barrel rifle and plan to sell it later you may be waiting a while.  You must follow all NFA rules which require the weapon to be transferred to another civilian.  This limits the amount of people who will be able or willing to purchase your weapon. The second hand market for NFA weapons is not great unless you a dealing in something fully automatic.

Unfortunately, anytime the media discovers a topic they can use to blow out of proportion they quickly exploit it.  One of the best examples is civilian ownership of machine guns.  Everyone has seen machines guns in use on TV, movies, and in video games. They are cool, but the fact is owning one is extremely expensive and somewhat hard to do. 

Machine guns are defined as any weapon that fires more than 1 round with each trigger pull. This is commonly known as fully-automatic, but also qualifies with a 3-round burst function, or even 2 rounds from weapons like an H&K UMP. Machine guns CANNOT be manufactured by any person, company, or government agency and then sold to civilians. Due to the GCA (Gun Control Act) and NFA (National Firearms Act) machines guns are highly restricted.

These laws prohibit civilian ownership of any US made machine guns after 1986. Further more, state laws may prohibit machine guns, so you would need to check your local and state laws before trying to purchase one. A US made machine gun of pre-1986 is usually a Colt M-16 (called a preban M-16). These are highly sought after by collectors; and because of the pre-1986 law, the supply of these guns continues to shrink as they are destroyed, lost, stolen, etc. This only increases the price of these guns over time. At the time this article was written, a Colt M-16 machine gun sells anywhere from $15,000 to $20,000. WOW! Although, there are alternatives to buying a complete machine gun.

Certain devices called lightning links will also transform certain firearms into machine guns. They are not much cheaper and are just as hard to find. Another device called an RDIAS (registered drop-in auto sear) will turn any AR-15 into a fully automatic weapon. They are just as expensive, if not more, than a pre-ban M16.

So you shell out the cash and buy a machine gun, if you have the money to spend is it really that easy? The answer of course is no. Along with a hefty price tag, the GCA requires any civilian to register the machine gun with BATFE and pay a $200 tax. This process is somewhat cumbersome, and requires a wait time ranging anywhere from 3-7 months for the paperwork to be processed and tax stamp issued. When this process is required, it is referred to as a Class 3 item or NFA item. There are several different ways to complete the paperwork, but that is covered in another article. You cannot take ownership of the item until all that is completed and processed by the BATFE.

Let's look at the total cost of ownership for a Colt M-16. Let's say Joe buys a machine gun from a Class 3 dealer for a cost of $15,000. He would then need to complete the NFA paperwork. One of the most common ways to complete this is using a revocable living trust specifically for firearms. These are usually prepared by attorneys and cost between $200-$500. The NFA paperwork would include the standard forms, a copy of the firearm trust, and a check for $200 which covers the cost of a tax stamp.

Assume a standard tax rate of 6.5% (because that what we have in Florida) combined with the rest of the fees it comes to:

$15,975 : Cost of Machine Gun (cost may be closer to $20,000)
    350 : Cost of Firearms Trust
    200 : Cost of Tax Stamp

$16,525 Total Cost of Ownership

Let's not forget the NFA paperwork/tax stamp will take 3-7 months for processing. During those months, the Class 3 dealer will retain possession of the machine. So as you can see, the process and cost of owning a machine gun is not cheap or easy to do. As time goes on, the supply of transferable machine guns will shrink and continue to drive the price and demand for these weapons. If you plan to shoot a machine gun at your local range, they will probably request to see a copy of your NFA paperwork, so you will be required to carry that with the weapon.

Considering the current economy, buying a machine gun is probably one the best investments money can buy. The problem is freeing up the $16,000+ for it.

The answer is usually yes.  Most modern suppressors are modular meaning you can swap out different pistons or adapters for various threading and/or caliber.  Keep in mind that shooting a 9mm bullet through a .45 suppressor will not be as effective as a traditional 9mm suppressor.  By design, a .45 caliber suppressor will be built to suppress a .45 bullet.  The smaller diameter, faster travel time, and different bullet weight of a 9mm will affect the suppression.

Older suppressors may not have the ability to adapt different calibers and/or threading.  Your best bet is to check with the manufacturer of each suppressor.  I own an AAC Ti-rant 45 which has different pistons for 9mm and .45 ACP.  I have used it with my USP-SD and it works great. I have also used the Ti-rant 45 with several .22 hosts and it works great. Although I have to switch to subsonic ammo to get a reasonable performance.  It's a much cheaper alternative to buying another 9mm suppressor and waiting another 7 months for the paperwork to come back. The pistons shown below are for use with the Tirant 45.

This same concept also applies to other calibers. Rifle rounds like the 7.62x39 and 5.56 can be fired from the same suppressor. Of course, NEVER try to fire a larger caliber than the suppressor you have. Some shooters with multiple suppressors will mark them accordingly so they do not grab a 5.56 can and accidentally attach it to a weapon shooting 7.62x39.