I have four of the Sightmark laser boresights - the 12 gauge, 9mm, 7.62x39mm, and .223.I had used a Bushnell laser boresight before getting this set of chamber loaded laser boresights. The Bushnell is the kind that you stick into the muzzle end of the gun, fitted with an arbor matched to the barrel size. The problem with this device was that the arbors never fit quite right - they were always either too tight or too loose, plus you had to try to seat the body of the boresight in the center of the opening of the barrel (or muzzle brake) in order to properly align the center axis of the boresight with the center axis of the barrel. This rarely if ever happened just right as the slightest tilt in the seating of this boresight in the barrel would take it off the center axis so that rotating the laser would cause the laser light to mark out a small circle that was several MOA in diameter rather than being dead centered. Ugh.Of course with these chamber loaded laser boresights, there is also the potential for the lasers to be mis-aligned off the center axis. Plus there is the potential for the boresight to not fit the chamber properly. The key is to find a brand that has a measure of quality control so that you aren't stuck with these problems.I read through a lot of reviews before deciding on Sightmark. It is definitely not the cheapest out there. There are so many different brands and generic versions of these chamber loaded laser boresights now that almost look alike, with many much cheaper than the Sightmark. However, it was clear from reading the reviews that as the prices went down, so did the quality control. Several of these cheaper brands came with complaints that the lasers were not aligned to the center axis (resulting in the laser painting a circle as it rotates in the chamber) or that they did not fit properly in the chambers and so would either get stuck or wiggle.I have this theory that the really cheap laser boresights must be factory seconds or some such.I considered the Osprey line with the adjustable arbors, which saves money by allowing you to use one laser bore sight for different calibers. But, the reviews indicated that hassling with arbors would have an effect on accuracy and reliability.I almost got the Firefield, which seemed to have a similar number of decent reviews. Ultimately I decided against it because the entire line is made of an aluminum shell, and I think the brass shell of the Sightmark will hold up better.It should be noted, however, that the 12 gauge Sitemark is made out of aluminum, unlike the others, and is only painted a brass color.The biggest complaint against all of these laser boresighters are the frequent DOA batteries and poor battery life. The Sightmarks all come with alkaline button cell batteries. The small caliber ones (9mm, 7.62mm, .223) come with two AG5 batteries while the 12 gauge came with three AG13 (LR44) batteries.To get you all confused, the printed information on the back of the packaging for the 7.62mm and .223 says you can use 3 AG3 batteries in place of the 2 AG5 batteries (since the AG3 is a thinner battery with the same 7.9mm diameter), while the printed info for the 12 gauge says you can use 2 AG5 or 3 AG13 batteries. This is flat out incorrect since 2 AG5 batteries are too small to fit into the 12 gauge Sightmark battery chamber.Confused? Go to wikipedia and look up the article "List of battery sizes". Scroll down to the section titled "Silver oxide and alkaline cells". The table lists all the different names for each battery, their dimensions, and their mAh power rating. Note how the silver oxide batteries contain 1-1/2 to two times as much energy as the alkaline batteries.So the key is to go out and get some silver oxide button batteries, size SG5 for the AG5 powered Sightmarks and SG13 for the AG13 powered lasers. And don't go to your nearest retail store, since that will cost you an arm and a leg. Google shop for these batteries online. I found two Made in China "brands" one called Vinnic and another by Chung Pak Battery Works which were pretty cheap. The names aren't important, since I'm sure they will change rapidly. Just remember to get some silver oxide batteries online at the same time you are getting these Sightmark bore sights.How well did these Sightmarks work? The very first time I used one (the 7.62x39mm), I plugged in the included AG5 alkaline batteries and the resulting laser light was exceedingly weak. My initial reaction was that this was a bad laser. Then I remembered the silver oxide batteries I had gotten. When I switched out the batteries, the laser suddenly glowed brightly. Later, when this set of silver batteries finally wore down, the laser light faded again. That's a sign to change batteries.The larger AG13 alkaline batteries for the 12gauge shotgun boresight worked reasonably well and are still working as of this writing. Not surprising, since these batteries pack a lot more energy than the small AG5 batteries.As many have noted, the Sightmarks do not have an on-off switch. Once the batteries are screwed in the laser light is on! This is just as well, since the bore sights have to flush fit inside the chamber of the barrel and endure having the bolt and firing pin slammed into them. But you do have to have a plan of action ready so as to not run down your batteries. With one pair of SG5 silver oxide batteries, I was able to adjust the sights for about four guns, each taking about 5-10 minutes. So I am guesstimating a battery life of about 30 minutes with a fresh pair of SG5 silver batteries (the Sightmark packaging claims up to an hour of battery life - hah!).All of the Sightmarks have fit in well, none have gotten stuck (so far), and none have had off-center lasers.These chamber loaded Sightmark boresighters make it easy to dial in a scope, and for fixed sights, they help confirm the aim points. They are much better than the muzzle loaded bore sights, although the total cost is a lot more since they are ammo caliber specific.