An added problem was that many of the weapons carried by employees of private security firms weren't registered under companies' names.
Instead, the weapons had private registrations that made the task of evaluating the firms' arsenals more difficult, said an advance research note in the forthcoming Small Arms Survey 2011, released in Geneva, Switzerland.
The survey's findings were summarized in a report that drew attention to the excessive stockpiles of firearms in the private security firms sector in Latin America. The Small Arms Survey is published annually by the Graduate Institute of International and Development Studies.
Citing findings on the private security companies' firearms holdings, their scale and variation across regions, the center said information on private security companies and their weapons holdings remained scarce and private security contracts were harder to track down.
It said that countries that maintain firearm registration systems don't always distinguish between civilian-held firearms and weapons held by those working for private security firms.
In some countries, PSC employees can carry their personal weapons while on duty, further complicating accounting. In other cases, personnel have been reported to carry illegal and unrecorded firearms.
The private security companies came into the glare of publicity after their extensive deployment in Afghanistan and Iraq. At least 35 private security firms are still active in Afghanistan. Both the full extent of their arsenals and their cash turnover remain unknown.
In Latin America, private security firms saw their business grow in Brazil, Venezuela, Argentina and other countries with high crime rates, data from news media indicated.
Although four Latin American countries -- Argentina, Brazil, Guatemala and Peru -- are among nations that disallow the use of automatic firearms by private security firms, news reports consistently suggested their use in incidents involving private security firms, analysts said.
The Small Arms Survey 2011 says an examination of PSC firearms holdings merits greater scrutiny.
"Their numbers may represent but a small fraction of total civilian stockpiles, yet the more interesting question is how they relate to certain state security sector holdings, and what does this say about the state's role in upholding law and order.
"Moreover, oversight and transparency regarding their stockpiles is often slim at best," said the report.
"In many countries, official standards for the management and safeguarding of PSC weapons, as well as for the training of PSC personnel, are non-existent. Confidentiality of internal PSC procedures also makes an evaluation of industry standards and performance particularly difficult," added the report.